iv MODERN EVOLUTION 221 



This argument has suggestive illustration in the 

 fifth chapter of the Origin of Species. Mr. Darwin 

 there refers to a remark to the following effect made 

 by Mr. Waterhouse : * A part developed in any species 

 in an extraordinary degree or manner in comparison 

 with the same part in allied species tends to be highly 

 variable! This applies only where there is unusual 

 development. ' Thus, the wing of a bat is a most 

 abnormal structure in the class of mammals ; but 

 the rule would not apply here, because the whole 

 group of bats possesses wings ; it would apply only 

 if some one species had wings developed in a 

 remarkable manner in comparison with the other 

 species of the same genus.' And when this excep- 

 tional development of any part or organ occurs, we 

 may conclude that the modification has arisen since 

 the period when the several species branched off 

 from the common progenitor of the genus ; and this 

 period will seldom be very remote, as species rarely 

 endure for more than one geological period. 



How completely this applies to man, the latest 

 product of organic evolution. The brain is that 

 part or organ in him which has been developed ' in 

 an extraordinary degree, in comparison with the 

 same part ' in other Primates, and which has become 

 highly variable. Whatever may have been the 

 favouring causes which secured his immediate pro- 

 genitors such modification of brain as advanced him 

 in intelligence over ' allied species,' the fact abides 

 that in this lies the explanation of their after-history ; 

 the arrest of the one, the unlimited progress of the 

 other. Increasing intelligence at work through vast 

 periods of time originated and developed those social 



