But, to look at the matter in another light. Mr. Darwin maintains natural selection to be "the most important but not the exclusive means of modification."
For "natural selection" substitute the words "survival of the fittest," which we may do with Mr. Darwin's own consent abundantly given.
To the words "survival of the fittest" add what is elided, but what is, nevertheless, unquestionably as much implied as though it were said openly whenever these words are used, and without which "fittest" has no force--I mean, "for the conditions of their existence."
We thus find that when Mr. Darwin says that natural selection is the most important, but not exclusive means of modification, he means that the survival in the struggle for existence of those creatures which are best fitted to comply with the conditions of their existence is the most important, but not exclusive means whereby the descendants of a creature, we will say, A, have become modified, so as to be now represented by a creature, we will say, B.
But the word "_circonstances_," so frequently used by Lamarck for the conditions of an animal's existence, contains, by implication, the idea of animals _which shall exist or not according as they fulfil those conditions or fail to fulfil them_. Conditions of existence are conditions which something capable of existing must fulfil if it would exist at all, and nothing is a condition of an animal's existence which that animal need not comply with and may yet continue to exist. Again, the words "animals" and "plants" comprehend the ideas of "fit," "fitter," and "fittest," "unfit," "unfitter," and "unfittest" for certain conditions, for we know of no animals or plants in which we do not observe degrees of fitness or unfitness for their "_circonstances_" or environment, or conditions of existence.
The use, therefore, of the term "conditions of existence" is sufficient to show that the person using it intends to imply that those animals and plants will live longest (or survive) and thrive best which are best able to fulfil those conditions. Hence it implies neither more nor less than what is implied by the words "struggle for existence, with consequent survival of the fittest"--that is to say, if we hold the complying with any condition of life to which difficulty is attached to be part of "the struggle" for life, and this we should certainly do.