388 OUTLINES OF EVOLUTIONARY BIOLOGY 



diverging twigs destroying the less vigorous the dead and lost 

 branches rudely representing extinct genera and families." 



The same paper also contains a sketch of the supplementary 

 theory of sexual selection, which will be seen to agree very closely 

 with the paragraph from Erasmus Darwin's " Zoonomia " quoted 

 in the last chapter : 



" Besides this natural means of selection, by which those 

 individuals are preserved, whether in their egg, or larval, or 

 mature state, which are best adapted to the place they fill in 

 nature, there is a second agency at work in most unisexual 

 animals, tending to produce the same effect, namely, the struggle 

 of the males for the females. These struggles are generally 

 decided by the law of battle, but in the case of birds, apparently, 

 by the charms of their song, by their beauty or their power of 

 courtship, as in the dancing rock-thrush of Guiana. The most 

 vigorous and healthy males, implying perfect adaptation, must 

 generally gain the victory in their contests. This kind of selec- 

 tion, however, is less rigorous than the other ; it does not require 

 the death of the less successful, but gives to them fewer 

 descendants." 



The publication from which these quotations are taken itself 

 consists of extracts from Charles Darwin's manuscripts, selected 

 with a view to explaining, as clearly as possible, his theory of 

 natural selection, and is therefore especially suitable for citation. 

 In the following year (1859) the author's classical work, " The 

 Origin of Species by means of Natural Selection," made its 

 appearance, the first of that notable series of volumes on 

 philosophical biology which have made his name so famous. 

 In these works both the general theory of Evolution and the sub- 

 sidiary theory of Natural Selection are elaborated and supported 

 by an immense body of evidence drawn from published records 

 and personal observations, and so successfully was this done that 

 in a comparatively few years these theories met with general 

 acceptance on the part, not only of scientific men, but also 

 of the educated public. It must not be forgotten, either, that 

 Charles Darwin applied the doctrine of organic evolution 

 in a fearless and uncompromising manner to the origin of the 

 human race. 



Dr. Wallace's contribution to the Linnean Society symposium 

 of 1858 was entitled " On the Tendency of Varieties to depart 



