The facts

Survival of the fittest (SOTF) was coined by Herbert Spencer and first appeared in print in his book, Principles of Biology (1864).

… those individuals whose functions are most out of equilibrium with the modified aggregate of external forces, will be those to die; and that those will survive whose functions happen to be most nearly in equilibrium with the modified aggregate of external forces. But this survival of the fittest, implies multiplication of the fittest. Out of the fittest thus multiplied, there will, as before, be an overthrowing of the moving equilibrium wherever it presents the least opposing force to the new incident force. And by the continual destruction of the individuals that are the least capable of maintaining their equilibria in presence of this new incident force, there must eventually be arrived at an altered type completely in equilibrium with the altered conditions. <emphasis added>

Herbert Spencer, Principles of Biology (1864), pg. 444

Pretty harsh it would seem that “those individuals whose functions are most out of equilibrium with the modified aggregate of external forces, will be those to die.” But was Spencer suggesting those who are “most out of equilibrium,” i.e. the sick, the stupid, even the unlucky, should be caused to die? No. Nowhere does Spencer argue that humans should do harm to other humans to make the species, the nation, or society more fit, just the opposite. He believed that once humans became civilized, the State acting to do harm to humans, e.g. war, democide, genocide, eugenics, etc., becomes untenable with respect to social fitness because it violates our evolved moral sentiment that comes with civilization.

For Spencer, SOTF wasn’t some kind of call to action to weed out the weak, but simply an objective statement of truth regarding nature. SOTF is a value neutral observation about the general nature of nature that applies to all systems in nature. How we respond to this axiomatic truth is a separate question.

But where the external changes are either such as are fatal when experienced by the individuals, or such as act on the individuals in ways that do not affect the equilibrium of their functions ; then the re-adjustment results through the effects produced on the species as a whole—there is indirect equilibration. By natural selection or survival of the fittest—by the preservation in successive generations of those whose moving equilibria happen to be least at variance with the requirements, there is eventually produced a changed equilibrium completely in harmony with the requirements. And thus it results that those universal laws of the redistribution of matter and motion, which are conformed to by evolution in general, are conformed to by organic evolution.

Herbert Spencer, Principles of Biology (1864), pg. 474

Note that in the above quote, Spencer wrote “And thus it results that those universal laws of the redistribution of matter and motion, which are conformed to by evolution in general, are conformed to by organic evolution.” <emphasis added>. Here Spencer shows that SOTF wasn’t exclusive to organisms or entire species but all systems, organic and inorganic (e.g. climates, oceans, forests, societies, businesses, nations, planetary, etc.)–every complex thing in nature. Thus, SOTF is a more universal principle than natural selection.

It was Darwin who chose to use SOTF as an alternative to natural selection in his 5th edition of Origins at the suggestion of a mutual acquaintance. Spencer had nothing to do with that decision as many people seem to believe. In his letter to A. R. Wallace (5-Jul-1866), Darwin agrees to use SOTF but notes that Spencer himself continued to use “natural selection,” very likely because he understood SOTF to have a different meaning.

My dear Wallace

I have been much interested by your letter which is as clear as daylight.2 I fully agree with all that you say on the advantages of H. Spencer’s excellent expression of “the survival of the fittest.”3 This however had not occurred to me till reading your letter. It is, however, a great objection to this term that it cannot be used as a substantive governing a verb; & that this is a real objection I infer from H. Spencer continually using the words natural selection.

With many thanks for your interesting letter, believe me, | my dear Wallace, yours sincerely | Ch. Darwin

Darwin Correspondence Project, “Letter no. 5145,” accessed on 11 October 2019,

Spencer considered SOTF in Newtonian terms. Newton’s First Law of Motion goes, “A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force.” Spencer thought of any system in nature, e.g. an organism, a species, ecosystem, planetary, galaxy, etc., similarly as “a system in equilibrium will tend to remain in equilibrium until acted upon by a disrupting external force causing it to adapt or cease to exist.” He termed the state of a system at rest, i.e. in steady-state equilibrium, as a “moving equilibria.” Thus, SOTF to Spencer was more about physics than politics, society or economics. In other words, SOTF is value neutral.

In his autobiography, Spencer questions using SOTF to replace the term “natural selection,”

“Natural selection as ordinarily described, is not comprehended in this universal redistribution. It seems to stand apart as an unrelated process. The search for congruity led first of all to perception of the fact that what Mr. Darwin called ‘natural selection,’ might more literally be called survival of the fittest. But what is survival of the fittest, considered as an outcome of physical actions? The answer presently reached was this:—The changes constituting evolution tend ever towards a state of equilibrium. On the way to absolute equilibrium or rest, there is in many cases established for a time, a moving equilibrium—a system of mutually-dependent parts severally performing actions subserving maintenance of the combination.”

Herbert Spencer, An Autobiography Vol. 2 (1904)

Common misconceptions

The table below contains several popular misconceptions surrounding SOTF from the Wikipedia article (as of 10-Oct-2019). The phrase is well-known but almost universally misunderstood, at least with respect to Spencer’s intended meaning.

Myth (from Wikipedia)Truth
"Herbert Spencer first used the phrase, after reading Charles Darwin's On the Origin of Species, in his Principles of Biology (1864), in which he drew parallels between his own economic theories and Darwin's biological ones: 'This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called 'natural selection', or the preservation of favoured races in the struggle for life.'"The very first paragraph of the Wikipedia article gets it completely wrong. Spencer's SOTF had nothing to do with economics per se nor was it economics that influenced him. The phrase was a result of Spencer trying to reconcile the theories of Darwinian natural selection and Lamarckian use-inheritance. He was unwilling to abandon Lamarck completely in favor of Darwin, so he considered the possibility that both could still be right by degree. SOTF was a product of Spencer considering the complexity of nature in that more than one mechanism could be responsible for evolution. And if so, almost as an open question, what are the implications to humanity? SOTF is really about surviving in a complex environment of disruptive forces that nature imposes on us.

The Wikipedia quote omits important context.
"While the phrase 'survival of the fittest' is often used to mean 'natural selection', it is avoided by modern biologists, because the phrase can be misleading. For example, survival is only one aspect of selection, and not always the most important. Another problem is that the word 'fit' is frequently confused with a state of physical fitness. In the evolutionary meaning 'fitness' is the rate of reproductive output among a class of genetic variants."It was none other than Darwin himself who took to calling natural selection SOTF by his 5th ed. Origin. This, because Darwin was never happy with "natural selection" since the phrase implies nature possessing a will, which could even imply the existence of God, something Darwin wouldn't have intended. But Darwin got SOTF wrong too. Spencer's SOTF is a universal principle that applies to all systems in nature, organic and inorganic. It's not exclusive to biological species. Spencer chose the unfortunate word "survival" which suggests organisms but he meant survival to mean continuation. Another way of expressing SOTF would be "adapt or die" with die meaning "cease to be."

"Survival is only one aspect of selection..." misunderstands the scope of the term. When referring to an organism, it does mean survival of that specific creature. But survival of the species means whatever strategy permits the species to be fit in nature. Thus, "survival" can mean mass reproduction, e.g. the African driver ant queen can produce 1,000,000 eggs a month, while human females can only have at most a few dozen babies in a lifetime. But reproduction is only one strategy that species use to survival in nature. SOTF only means the species with the best survival strategies given their environmental circumstances tend to survive.

For the human species, our best survival strategy has been cooperation in the exercise of individual human faculties of tool-use, cognition and language—not mass reproduction.

And does it really matter if "fit" is confused with physical fitness? Physical fitness is one aspect of being fit in nature The context of the word usage suggests what someone is talking about.
"The phrase can also be interpreted to express a theory or hypothesis: that 'fit' as opposed to 'unfit' individuals or species, in some sense of 'fit', will survive some test. Nevertheless, when extended to individuals it is a conceptual mistake, the phrase is a reference to the transgenerational survival of the heritable attributes; particular individuals are quite irrelevant. This becomes more clear when referring to Viral quasispecies, in survival of the flattest, which makes it clear to survive makes no reference to the question of even being alive itself; rather the functional capacity of proteins to carry out work.""Fit" means to be fit in nature, all things considered over time, therefore the test is nature itself. Thus, so long as you're alive you're fit, i.e. the moving equilibria that is you is still alive. But you become "unfit" when you are faced with death and are unable to adapt to avoid dying. Thus as an individual human, you want to keep yourself as fit as possible in mind and body, i.e. possess well-being, such that you don't approach death.

"Nevertheless, when extended to individuals it is a conceptual mistake..." Not so. An individual is itself a system in nature as is the species to which the individual belongs. SOTF applies to all systems in nature, whether an individual or the entire species.
"Interpretations of the phrase as expressing a theory are in danger of being tautological, meaning roughly 'those with a propensity to survive have a propensity to survive'; to have content the theory must use a concept of fitness that is independent of that of survival."This is a semantic sleight-of-hand. The words "natural selection" don't convey anything without definition. The theory is the theory and the label is just the label. SOTF isn't a theory but an axiomatic truth. What's to prove? Why did the dinosaurs go extinct? Because their species weren't fit in nature. Mammals survived, thus mammals were more fit than dinosaurs, all things considered over time.
"It has been claimed that 'the survival of the fittest' theory in biology was interpreted by late 19th century capitalists as 'an ethical precept that sanctioned cut-throat economic competition' and led to the advent of the theory of 'social Darwinism' which was used to justify laissez-faire economics, war and racism. However, these ideas predate and commonly contradict Darwin's ideas, and indeed their proponents rarely invoked Darwin in support.[citation needed] The term 'social Darwinism' referring to capitalist ideologies was introduced as a term of abuse by Richard Hofstadter's Social Darwinism in American Thought published in 1944."The term "social Darwinism" is nonsensical propaganda. It was used by European Marxists in the early 20th century then re-popularized by Hofstadter in the 40s as an intellectually dishonest attack on laissez-faire capitalism (LFC) as being "red in blood and claw, the strongest win." SOTF says nothing directly about LFC. The implication though is to ask, what economic model makes humans more fit in nature? The answer, which Marxists don't like, is LFC for many reasons which is why Spencer was a proponent of LFC. LFC is the logical consequence of equal freedom and Spencer believed equal freedom makes human society most fit in nature, i.e. the greatest well-being of the greatest number, all things considered over time.

Spencer had nothing to do with "social Darwinism" and neither did Darwin. Darwinian natural selection has to do with random mutations, i.e. mindless modification over time. Human society is mindful change over time.

"Russian anarchist Peter Kropotkin viewed the concept of 'survival of the fittest' as supporting co-operation rather than competition. In his book Mutual Aid: A Factor of Evolution he set out his analysis leading to the conclusion that the fittest was not necessarily the best at competing individually, but often the community made up of those best at working together."Kropotkin got it right that cooperation makes human society more fit in nature, but also wrong. He didn't understand the concept of peaceful, cooperative competition which is what LFC is based upon. LFC means cooperative competition to produce the greatest value for others. Think of any competitive sport. Players peacefully cooperate knowing that only one team can win. The result for the fans is to watch the teams vie to be the best thus bringing out the best in the sport.

Spencer’s formulation

Herbert Spencer was asked by F. Howard Collins, who was writing a book on Spencer’s Synthetic Philosophy at the time, to provide him with a simple explanation of his philosophy. Spencer responded with the outline reproduced below.

Even though Spencer didn’t explicitly use the term SOTF, the outline captures his philosophy regarding the nature of nature, he called his Doctrine of Evolution, which can be considered SOTF by another label.

Note that nowhere does he mention of capitalism, empire, colonialism, race superiority, or any of the nonsense haters and ne’er-do-wells attribute to the phrase or Spencer for that matter.

Some eighteen or more years ago, an American friend requested me, with a view to a certain use which he named, to furnish him with a succinct statement of the cardinal principles <that can be called “survival of the fittest”> developed in the successive works I had published and in those I was intending to publish. This statement, which I wrote out for him, and which has since made its appearance in England in a form giving it but little currency, I here reproduce to prepare the way for Mr. Collins’ epitome.

1. Throughout the universe in general and in detail, there is an unceasing redistribution of matter and motion.

2. This redistribution constitutes evolution where there is a predominant integration of matter and dissipation of motion, and constitutes dissolution where there is a predominant absorption of motion and disintegration of matter.

3. Evolution is simple when the process of integration, or the formation of a coherent aggregate, proceeds uncomplicated by other processes.

4. Evolution is compound when, along with this primary change from an incoherent to a coherent state, there go on: secondary changes due to differences in the circumstances of the different parts of the aggregate.

5. These secondary changes constitute a transformation of the homogeneous into the heterogeneous—a transformation which, like the first, is exhibited in the universe as a whole and in all (or nearly all) its details: in the aggregate of stars and nebulae; in the planetary system; in the earth as an inorganic mass; in each organism, vegetal or animal (von Baer’s law); in the aggregate of organisms throughout geologic time; in the mind; in society; in all products of social activity.

6. The process of integration, acting locally as well as generally, combines with the process of differentiation to render this change not simply from homogeneity to heterogeneity, but from an indefinite homogeneity to a definite heterogeneity; and this trait of increasing definiteness, which accompanies the trait of increasing heterogeneity, is, like it, exhibited in the totality of things and in all its divisions and sub-divisions down to the minutest.

7. Along with this redistribution of the matter composing any evolving aggregate, there goes on a redistribution of the retained motion of its components in relation to one another: this also becomes, step by step, more definitely heterogeneous.

8. In the absence of a homogeneity that is infinite and absolute, that redistribution of which evolution is one phase, is inevitable. The causes which necessitate it are these:—

9. The instability of the homogeneous, which is consequent upon the different exposures of the different parts of any limited aggregate to incident forces. The transformations hence resulting are complicated by—

10. The multiplication of effects. Every mass and part of a mass on which a force falls, sub-divides and differentiates that force, which thereupon proceeds to work a variety of changes; and each of these becomes the parent of similarly multiplying changes: the multiplication of them becoming greater in proportion as the aggregate becomes more heterogeneous. And these two causes of increasing differentiations are furthered by—

11. Segregation, which is a process tending ever to separate unlike units and to bring together like units—so serving continually to sharpen, or make definite, differentiations otherwise caused.

12. Equilibration is the final result of these transformations which an evolving aggregate undergoes. The changes go on until there is reached an equilibrium between the forces which all parts of the aggregate are exposed to and the forces these parts oppose to them. Equilibration may pass through a transition stage of balanced motions (as in a planetary system) or of balanced functions (as in a living body) on the way to ultimate equilibrium; but the state of rest in inorganic bodies, or death in organic bodies, is the necessary limit of the changes constituting evolution.

13. Dissolution is the counter-change which sooner or later every evolved aggregate undergoes. Remaining exposed to surrounding forces that are unequilibrated, each aggregate is ever liable to be dissipated by the increase, gradual or sudden, of its contained motion; and its dissipation, quickly undergone by bodies lately animate, and slowly undergone by inanimate masses, remains to be undergone at an indefinitely remote period by each planetary and stellar mass, which, since an indefinitely distant period in the past, has been slowly evolving: the cycle of its transformations being thus completed.

14. This rhythm of evolution and dissolution, completing itself during short periods in small aggregates and in the vast aggregates distributed through space completing itself in periods which are immeasurable by human thought, is, so far as we can see, universal and eternal—each alternating phase of the process predominating now in this region of space and now in that, as local conditions determine.

15. All these phenomena, from their great features down to their minutest details, are necessary results of the persistence of force, under its forms of matter and motion. Given these as distributed through space, and their quantities being unchangeable, either by increase or decrease, there inevitably result the continuous redistributions distinguishable as evolution and dissolution, as well as all those special traits above enumerated.

16. That which persists unchanging in quantity, but ever changing in form, under these sensible appearances which the universe presents to us, transcends human knowledge and conception—is an unknown and unknowable power, which we are obliged to recognize as without limit in space and without beginning or end in time.

Herbert Spencer, preface of F. Howard Collins, Epitome of the Synthetic Philosophy of Herbert Spencer, 5th ed., (Williams and Norgate, 1901) pp. viii-xi.