BIOLOGICAL INVESTIGATIONS. 137 



generally supposed. ^ Not only is this true for the smaller 

 phyla of animals, so far as known, but the now well-ascertained 

 fact that most existing types arose during earlier Paleozoic time 

 shows that it is probably a general law for the whole of the 

 animal kingdom. 



This excessively plastic period of evolution is followed by a 

 phase of slow changes in which the different branches of the 

 phylum acquire their slighter and less marked divergencies. 

 Thus, evolution is not invariable, but moves with extreme slow- 

 ness while each branch or phylum is in the .middle age or acme 

 of phylogeny. Finally, when retrogression begins, the pace is 

 again quicker, and the differences greater, owing to the abrupt 

 losses of progressive characters, and sometimes to the sudden 

 introduction of new and startling modifications. 



Many types, even at the present time, are still in the pro- 

 gressive stage of advancement, but fortunately the most spe- 

 cialized and complex of all types, the Mammalia, has nearly, if 

 not quite, reached an advanced phase of retrogression, and is 

 an admirable example of the quicker elaboration of differences 

 and the change of law in highly retrogressive types. 



The record of the immediate ancestor of man is either lost 

 or so difficult to trace in the embryo and younger stages that 

 observers have failed to come to any agreement, and yet he is 

 plainly descended from some simian predecessor. He is not 

 distinct from the most specialized of this group, the chimpan- 

 zee or gorilla, either by his bones or soft parts, so that in the 

 absence of his works a paleobiologist would not separate him 

 widely from them. All efforts to demonstrate adequate struc- 

 tural characters for the separation of man from the anthro- 

 poids have failed. The size of the brain cannot be admitted 

 as adequate, or any other differences of mass. The wide gulf 

 between man and his nearest relatives nevertheless becomes 

 plainer with every effort to close it up, and it is obviously 

 physiological. Even if all the intermediate links between him 

 and some anthropoid or other simian ancestor were to be found, 

 as they probably will be in time, the differences would remain 

 just as marked as before. 



1 " Phylogeny of an Acquired Characteristic," op. cit., pp. 366-371. 



