204 



THE THEOKY OF EVOLUTION 



new life conditions ; l for the rest the germ development 

 will proceed as hitherto. If now this unaltered re- 

 mainder is so constituted that there can be clearly 

 perceived in it a mode of evolution peculiar to a deter- 

 mined type, then we must regard the animal concerned 

 as a member, as a variation of that type. By what 



causes the deviations in 

 the completely formed 

 condition and in the 

 embryonic stages lead- 

 ing thereto are induced 

 can then under some 

 circumstances be 

 directly seen. If the 

 larva, for instance, 

 attaches itself firmly 

 and at once commences 

 the transformation 

 which strikes us in the 

 perfect animal, then 



FIG. 39. BARNACLE IN ITS FORMS OF ,1 -j- r r,- ncj fnrma 



DEVELOPMENT. was 



tion formerly caused 



for the first time by transfer of the animal to a sessile 

 mode of life. 



1 With regard to the time in which the separate tissues and organs 

 became differentiated during the embryonic development, there are often 

 great differences in the separate groups of animals. Biologists define 

 two great groups, the mosaic and the regulation ova, according to the earlier 

 or later commencement of the differentiation and the therewith connected 

 greater or lesser facility of the ' regulation ' (restitution) in the embryos 

 produced from the eggs. 



