466 THE EVOLUTION OF LIFE. 



such smaller rapidity as to make continuance of the rudi 

 ments of them comprehensible. 



131 132. So that while the embryonic law enunciated 

 by von Baer is in harmony with the hypothesis of evolu 

 tion, and is, indeed, a law which this hypothesis implies, the 

 nonconformities to the law are also interpretable by this 

 hypothesis. 



Parallelism between the courses of development in species 

 allied by remote ancestry, is liable to be variously modi 

 fied in correspondence with the later ancestral forms passed 

 through after divergence of such species. The substitution 

 of a direct for an indirect process of formation, which we 

 have reason to believe will show itself, must obscure the 

 embryonic history. And the principle of economy which 

 leads to this substitution produces effects that are very irregu 

 lar and uncertain in consequence of the endlessly varied con 

 ditions. Thus several causes conspire to produce deviations 

 from the general law. 



Let it be remarked, finally, that the ability to trace out 

 cmbryologic kinships and the inability to do this, occur just 

 where, according to the hypothesis of Evolution, they should 

 occur. We saw in lOOa that zoologists are agreed in group 

 ing animals into some 17 phyla Mollusca, Arthropoda, 

 Echinodermata, &c. each of which includes a number of 

 classes severally sub-divided into orders, genera, species. All 

 the members of each phylum are so related embryologically, 

 that the existence of a common ancestor of them in the 

 remote past is considered certain. But when it comes to the 

 relations among the archaic ancestors, opinion is unsettled. 

 Whether, for instance, the primitive Chordata, out of which 

 the V ertcbrata emerged, have molluscan affinities or anne- 

 lidan affinities, is still a matter in dispute. With regard to 

 the origins of various other types no settled conclusions are 

 held. Now it is clear that on tracing down each branch of 

 the great t genealogical tree, kinships would be much more 



