BIOLOGICAL EVOLUTION 63 
The first difficulty is concerning “ the absence or rarity of 
transitional varieties,” and his answer is: ‘‘ As natural selection 
acts solely by the preservation of profitable modifications, each 
new form will tend in a fully-stocked country to take the place of, 
and finally to exterminate, its own less improved parent-form and 
other less favored forms with which it comes into competition.” ! 
In discussing the difficulty of explaining neuter insects, Darwin 
formulates the doctrine of selection on the basis of utility to the 
species rather than to the individual.? 
In reply to criticism by Mr. Mivart, Darwin takes issue with all 
who believe in mutations, appealing as usual to experience under 
domestication.® 
Instincts are discussed at length and their origin explained in 
the same way as other useful characters, — by natural selection.‘ 
We find further contributions to our doctrine of adaptation in 
Darwin’s Descent of Man though here he was preceded by other 
writers. In this work we are shown how the various mental 
qualities so highly developed in man have descended or “ as- 
cended ” from rudiments to be found in the lower orders.® 
Emotion, imitation, attention, memory, imagination, reason, 
the use of tools, even language are thus evolved. All of these, — 
with many others such as self-consciousness, individuality, 
abstraction, general ideas, sense of beauty, religion, — are the 
1 Origin of Species, p. 134. 2 [bid., pp. 230 f. 
3 Ibid., pp. 202 f. Recent experiments by De Vries, Bateson and others, how- 
ever, indicate to their satisfaction that nature does take leaps, “ Natura facit 
saltum.”” Cf. Walter, Genetics, chs. IV, VII, and VIII. 
4 Tbid., ch. VIII. Professor T. H. Morgan takes issue with his conclusions 
concerning the development of such instincts as that of slave-holding among certain 
species of ants. ‘‘ We must not forget,” says Professor Morgan, “ that it is not 
enough to show that a particular habit might be useful to a species, but it should 
also be shown that it is of sufficient importance, at every stage of its evolution, 
to give a decisive advantage in the ‘ struggle for existence.’ For unless a life and 
death struggle takes place between the different colonies, natural selection is 
powerless to bring about its supposed results. And who will be bold enough to 
affirm that the presence of slaves in a nest will give victory to that colony in com- 
petition with its neighbors? Has the history of mankind taught us that slave- 
making countries have exterminated countries without slaves?’ His conclusion 
is that the instinct was a mutation and that the species practising it survived 
because it was not so disuseful as to lead to extermination. 
5 Articulate language, however. is peculiar to man. Descent of Man, p. 52. 
