336 BAUR. 



during the later period. TJie disposition to increase the number 

 must be, therefore, in the germ itself. The question is then, Is 

 the increase of the number of segments "accidental," and are 

 forms which show this "accidental" increase of segments pre- 

 served through natural selection ; or is this tendency to increase 

 the number of segments common to all individuals, not appear- 

 ing accidentally, but rather as the result of a definite stimulus ? 

 I can only adopt the latter possibility. 



But how is it in this case .'' Many animals increase the length, 

 that of the neck for instance, not by addition of new segments, 

 but simply by elongating the single segments present. For 

 instance, the Giraffe among mammals, Chelys, Chelodina, Diro- 

 chelys, Hydromedusa among the tortoises. How is it that the 

 Plesiosaur elongates its neck by adding new segments, and the 

 Tortoise by stretching the single segments .'' Here we are before 

 a difficult question. It is clear the long neck of the Giraffe 

 and of some of the Tortoises develops during the evolution of 

 the animal, but this tendency must be potentially in the germ. 

 It is again the tendency to elongate the neck which is impressed 

 on the germ, but in a different way. An interesting case of 

 addition of segments is offered by the Sirenians. The Sirenians 

 are the only mammals, the Cetaceans excepted, which show an 

 increase of phalanges (four instead of three) ; a fourth phalange 

 is added in some digits at the distal end, but this takes place 

 during the postembryonic life of the animal, the embryo having 

 only three phalanges. 



I am not able at present to give any explanation for these 

 phenomena, but I thought it worth while to mention them in 

 this connection. 



