26o BIOLOGICAL LECTURES. 



only by a minute study of the fossils themselves. Let us 

 assume, however, that the conclusions so far reached are well 

 established and consider' some of the consequences which have 

 an important bearing upon evolutionary philosophy in general. 

 (i) We see, in the first place, that parallelism and conver- 

 gence of development are very real phenomena, and that they 

 have played a highly important part in the course of evolution. 

 Many morphologists now accept this mode of development 

 unreservedly, but some still reject it altogether, or regard it 

 as something unusual and exceptional. However that may 

 be, the well-defined and established phyla of extinct mammals 

 demonstrate the actuality of these modes of development be- 

 yond peradventure. Shufflle the cards as we may, we cannot 

 arrange them so as to bring out any other result. One is 

 sometimes tempted to believe that the number of possible 

 tooth-patterns or osteological structures must be limited, so 

 often are the same ones repeated in different phyla. In many 

 instances we are able to follow out the history of a dental or a 

 skeletal structure, step by step, from its point of origin to its 

 final completion, and to show that the same structure has been 

 independently attained over and over again. So far as single 

 structures are concerned, this is now an old and familiar story ; 

 the spout-shaped odontoid process of the axis, which is so 

 common among hoofed animals, the double bicipital groove on 

 the humerus of horses, camels and giraffes, have often been 

 pointed out as examples of this mode of development. Only 

 of late, however, have we been in a position to prove that the 

 entire structure may be so modified in two different groups as 

 to keep parallel courses through long periods of time. One of 

 the most striking instances of this is offered by the history of 

 the cats. The family comprises two subfamilies, the Fclince, 

 or true cats, and the Machaiivdontince, or sabre-tooth cats ; 

 these two groups have been separate, at least since early 

 Oligocene times, when the principal family characteristics had 

 not yet appeared, and through the Miocene and Pliocene and 

 into the Pleistocene they follow parallel courses, the final genus 

 of one series, Sviilodon, being almost identical with the terminal 

 genus of the other, i.e., Felis. 



