IO6 EVOLUTION OF TO-DAY. 



kingdom, and many of the smaller ones. The in- 

 vertebrates in general, owing to their comparatively 

 simple organization, had already reached a condition 

 where slight variations produce only twigs, and the 

 development has since then been much slower. 

 The vertebrates, however, developed more slowly, 

 and did not reach a similar condition until much 

 later. The evidence indicates, in short, that the 

 power of divergence of any type is not unlimited. 

 Each has a certain place to fill in nature, and hav- 

 ing once approximately filled it, the future develop- 

 ment is of such a sort as to produce twigs rather 

 than branches ; and further, the evidence shows that 

 by the close of the Silurian, many of the inverte- 

 brate divisions had reached such a condition, while 

 the vertebrates have passed through nearly all of 

 their development since that time. To this latter 

 group, therefore, must we look for the bulk of our 

 evidence for evolution. 



In this connection it has frequently been urged 

 that the existence of any animal unmodified since 

 the early Silurian times, is a grave difficulty for evo- 

 lution. The existence of Lingula (a small bivalve 

 shell) with almost no change since the earliest fossil- 

 iferous rocks, has been regarded as a serious obstacle 

 for Darwin. But this idea comes from a misunder- 

 standing. Time does not necessarily imply change, 

 even upon the theory of evolution. Those organ- 

 isms become modified which are not in harmony 

 with their conditions, and consequently it requires 

 change of condition to produce change of organic 

 structure. Now Lingula is a very simple animal, 



