78 EVOLUTION OF TO-DAY. 



which is called embryonic or ontogenetic descent. 

 And this is a factor of which almost nothing is 

 known, and whose limits are as yet undrawn. One 

 naturalist, as will appear later, is inclined to make 

 this the all-important factor in heredity. But we 

 know too little of the real value of this hypothesis to 

 justify us as yet in admitting that it is sufficient to 

 clear up the vexed problem of serial homology. 

 Evidently this new factor will not explain homologies 

 between different animals, any more than the 

 ordinary theory of descent will account for serial 

 homology, although both are explained by descent. 

 If we can accept this theory the descent hypothesis 

 becomes logical as far as homologies are concerned. 

 If not, we must wait for another ; at best, we must 

 admit that the subject is still a perplexing one. 



Rudimentary Organs. 



An important feature in connection with the classi- 

 fication of animals and plants, is the series of organs 

 known as rudimentary organs. They have always 

 been an object of interest, for, utterly functionless, 

 their presence seems entirely unaccounted for unless 

 they are assumed to be rudiments of organs pre- 

 viously of value. Every one is acquainted with 

 examples. The coccyx of the human skeleton repre- 

 sents the tail of other mammals ; the so-called splint 

 bones of the horse's feet are the remains of the lost 

 fingers and toes ; the rudimentary legs of the boa- 

 constrictor, which do not protrude through the skin ; 

 the rudimentary teeth in the whale, which never 

 pierce the jaw so as to become in the slightest 



