EVOLUTION 



717 



progressively less like each other. Similarly, adult polychsetes 

 and molluscs have not much resemblance, but the trochophore 

 larva is common to both (p. 178), and earlier still the spiral 

 cleavage of the egg is almost identical in the two groups. We 

 may sometimes then be able to find relationships in the embryos 

 which are not apparent in the adults ; the sessile barnacles, and 



Fig. 551. — A dissection of the human ear to show the useless vestigial muscles. — 



From Gray. 



even more such parasites as Sacculina (p. 221), are hardly recog- 

 nisable as Arthropods, but their larvae are clearly Crustacea. 

 The form in which the biogenetic law is sometimes stated, that 

 ontogeny recapitulates phylogeny, or that in the course of its 

 life an animal climbs up its family tree, cannot be maintained, 

 but embryology does bring to light many transient vestigial 

 structures, such as the gill sHts of terrestrial vertebrates. 



8. What is generally called the evidence from classification is 

 also an extension of that from comparative anatomy, as the fact 

 that animals can be classified at all depends on their similarity 

 of structure. The difficulty of deciding on the limits of many 



