V. The Affinities of Animals, etc. 57 



Taensible, see p. 40, " Rudimentary Organs " ; p. 52, some plienomena 

 in Embryology ; and also the Sections followingj VI. to VIII. 



Whilst the doctrine of descent has heen talked about for a long 

 time, it is only in the latter pai-t of the present century that it has 

 gained general acceptance, and this is primarily due to Charles Dai-win 

 (1809 — 1882), especially to his work, "The Origin of Species," which first 

 appeared in 1859. The chief value of Darwin's work appears to consist 

 in his having brought together facts from all sides, demonstrating the 

 necessity for the acceptance of the theory. He has proved that a large number 

 •of facts of Zoo-geography, of Geology, of Embryology : that rudimentary organs : 

 that the whole theory of the relations of animals remain unintelligible if a 

 ■doctrine of " Special Creations " is accepted. He has also, and this is of the greatest 

 importance, disproved the generally received dogma of the immutability of 

 .species, by an examination of the variations of animals in a domesticated as well 

 .as in the wild state. His theory as to the means by which the modifications 

 have been brought about ("Theory of Selection") appears to be of i-elatively less 

 importance, although this was a side of his woi-k upon which he bestowed the 

 greatest energy and the most extensive studies. Darwin considered that in 

 Nature, as in the breeding of domestic animals, a selection takes place : for 

 those individuals are best fitted for the struggle for existence, which are 

 distinguished by small favourable variations : and in this way development 

 advances, for the less well equipped animal goes to the wall, whilst the better 

 suiwives (Natural Selection). Whether this always occurs in Nature seems 

 by no means certain, and in any case there are many characters in animals which 

 are useless, and cannot therefore be explained by an appeal to Natui-al Selection. 

 Besides this factor, Dai-win acknowledged the more direct influence of the 

 •environment as a cause of variation ; but the whole question seems still too far 

 from solution to be treated more elaborately in a Text Book such as this. 



The way in which evolution has proceeded and is proceeding, is probably as 

 f oUows : the formation of varieties is the first step ; when a species has spread 

 over a large area, it develops differently at different places, the varieties* foi-med 

 become more sharply separated under favourable conditions and become distinct 

 species. A species develops to constitute a new genus if the difference 

 between it and the other species of the same genus becomes clearly defined ; if it 

 adapts itself, for example, to special conditions : by adaptation to a herbivorous 

 and digging life, the badger genus has developed from the genus mai-ten. A 

 genus differentiated in this way may aftei-wards divide into several species. By 

 continuous progressive development, and by the accumulation of variations, 

 new famihes, new orders, etc., may arise. Each new type is, originally, only 

 a specially differentiated species, but this species may split, later, into several, 

 ■each of which again may be the starting-point for new genera. From this 

 account, it may be supposed that the limits of the different genera, famihes, etc., 

 are never fixed : and in many cases this is so : in many instances it is quite 

 arbitrary whether one or more distinct species is reckoned as a separate genus 

 ■or not, or whether a genus is considered as the type of a separate family or 

 not. In other cases, on the other hand, the limit is shai-per, for many species 

 and genera, which might have obscured the boundary line have become extinct in 

 -course of time (Section VIII.). 



Homology, Analogy. — By the gradual modification of animals, 

 the individual parts of organisms have often altered very much in 

 course of time, not infrequently indeed an organ has lost its original 



* In many cases new varieties have actually been produced by the transportal of 

 ispecies to new localities. 



