126 MODERN IDEAS OF EVOLUTION 



state of some of these creatures, and in the adult 

 condition of one animal referred to this group 

 (Appendicularid)) they have a sort of swimming tail, 

 which is stiffened by a rod of cartilage to enable it to 

 perform its function, and which for a time gives them 

 a certain resemblance to the lancelet or to embryo 

 fishes ; and this usually temporary contrivance, 

 curious as an imitative adaptation, but of no other 

 significance, becomes, by the art of appearing and 

 disappearing/ a rudimentary backbone, and enables 

 us at once to recognise in the young ascidian an 

 embryo man. 



A second method characteristic of the book, and 

 furnishing indeed the main basis of its argument, is 

 that of considering analogous processes as identical, 

 without regard to the difference of the conditions 

 under which they may be carried on. The great 

 leading use of this argument is in inducing us to 

 regard the development of the individual animal as 

 the precise equivalent of the series of changes by 

 which the species was developed in the course of 

 geological time. These two kinds of development 

 are distinguished by appropriate names. Ontogenesis 

 is the embryonic development of the individual 

 animal, and is of course a short process, depending 

 on the production of a germ by a parent animal or 

 parent pair, and the further growth of this germ in 

 connection more or less with the parent or with pro 

 vision made by it. This is, of course, a fact open to 



