EMBRYOLOGY. 99 



paths leading to different grades of develop- 

 ment.* 



The same laws of embryonic development 

 hold good among lower organisms. Thus the 

 larvae of insects, which may be regarded in 

 one sense as foetal forms, resemble the early 

 stage of the lower groups of Annulosa, the 

 Annelida, and Myriopoda. The larvse of all 

 the Crustacea resemble each other very closely 

 on emerging from the egg, but afterwards 

 become differentiated in an analogous manner 

 to the foetal forms of the higher animals. f The 

 tadpole, which is the larva of the frog, re- 

 sembles an embryonic fish, and there is a 

 striking resemblance between the Protophyta 

 and pollen-grains. 



The law of progressive development in the 

 succession of organic forms on the earth will 

 account for resemblances between the embryos 

 of existing animals, and the adult forms of 

 extinct species. Thus, the Trilobites resembled 

 the larval forms of existing Crustacea, such as 

 Limulus ; and the most ancient fishes resembled 

 the embryos of existing fish. In the same 



* II. Spencer, " Biology," vol. i. pp. 143, 144. 

 t Carpenter, op. tit. p. 99. 



H2 



