380 THE PINEAL ORGAN 



tion in general, than it was at the close of the nineteenth century. It is, 

 we believe, now more generally appreciated that many of the striking 

 similarities which undoubtedly exist in certain of the more highly differ- 

 entiated animals which belong to otherwise dissimilar or divergent 

 classes are not necessarily to be regarded as evidence of a close relationship 

 between these classes ; and that if the environmental conditions remain 

 similar, functioning organs which are well adapted to their environment 

 and requirements may continue with little change for immensely long 

 periods of time. While, on the other hand, differences such as exist 

 between the more highly differentiated types of the principal phyla and 

 classes also require an immensely long period of time to evolve. More- 

 over, the differences which constitute class distinctions were in many 

 cases already established in the earliest known fossil representatives of 

 the classes concerned, e.g. to take a concrete instance, between the 

 Merostomes and Ostracodermi. This indicates that the period in which 

 the hypothetical common ancestral stock existed must have extended 

 many ages further back than that of which we have definite geological 

 evidence. 



