346 SPAEKS FROM A GEOLOGIST'S HAMMER. 



structure of the egg, simple cells, often, like some eggs, 

 capable of movement by means of prolongations of their 

 substance. There are some which attain to the morula 

 condition, and then are adult. Others pass to the planula 

 stage, and still others to the gastrula. Certain worms 

 (Turbellaria) represent a succeeding stage, as the Ascid- 

 ians are believed to picture a still later one. Thus on, 

 from the lancelet and the lampreys to the sharks, Am- 

 phibians, Monotremes, Marsupials, and Lemurs at the 

 bottom of the four-handed animals, we discover living 

 forms which stand forth in the museum of Nature as 

 pictures of the embryonic stages of the highest vertebrate. 



Finally, the embryonic series finds its parallel not only 

 in the embryonic history of other animals, and in the 

 adult forms of animals presented as we range up and 

 down the scale of life, but the succession of extinct types, 

 as far as we have read it, presents us with another par- 

 allel. 



Now, while we know the stages of the embryonic series 

 to stand derivatively related, it seems reasonable to infer 

 that the corresponding forms in the realms of actual and 

 extinct life are also derivatively related. It would appear, 

 at first view, that the nature of the derivation must be 

 fundamentally different in the two cases; but even this 

 does not impair the meaning of the fact that, in both 

 cases, we should have a material continuity from form to 

 form^ and this is all which evolution requires. On re- 

 flection, however, the mode of the continuity in the case 

 of the embryo appears substantially identical with the 

 assumed mode of continuity in the succession of geolog- 

 ical types. Ordinary embryonic development proceeds 

 through the multiplication and specialization of cells 



