i8o READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



■freely in the sea. However, these larval organs .... are never 

 properly functional, since no actually free-swimming larva is developed 

 but the embryo merely floats in the albuminous fluid of the cocoon. 



'^ A particularly beautiful example is offered by the whales in their 

 embryological development, which has been thoroughly studied by 

 Kukenthal. In the adult condition they show only the anterior 

 extremities, but in the embryo the posterior pair, with their skeletal 

 parts, are formed,but are afterwards completely atrophied. Although 

 they are mammals, in the adult condition they have absolutely no 

 covering of hair, since in their aquatic life another and more effective 

 protection against loss of heat is given by means of a thick layer of 

 blubber; only a few coarse bristles, partly with particular functions, 

 have persisted on a few parts of the body. But in the embryo a dense 

 covering of hair is formed, which is later transformed in a peculiar 

 manner and atrophied. Further, a series of whales have no teeth in 

 the adult condition, but only the well-known, eel-trap-like, horiiy 

 plates, from which whale-bone is produced. Nevertheless, in the 

 embryo there is a dentition of numerous teeth, which are, however, 

 resorbed, without ever piercing the gum."^ 



Throughout the great group of the ruminants, which includes the 

 oxen, buffaloes, bison, sheep, goats, antelopes, deer and giraffes, the 

 collar-bone is invariably lacking, since it is superfluous on account of 

 the exclusively locomotive manner in which the fore legs are employed. 

 In the embryo sheep the collar-bone is established and even, to some 

 extent ossified, but is subsequently resorbed and disappears entirely. 

 No doubt, the collar-bone will be found in many other embryo rumi- 

 nants, when the proper examination shall have been made, but its 

 demonstrated presence in the foetal sheep is sufficiently striking. In 

 the higher mammals the number of teeth was originally 44, or 11 on 

 each side of both upper and lower jaws, but in most of the modern or 

 existing groups of these higher mammals this number has been very 

 considerably reduced through the suppression of certain teeth. We 

 have every reason to believe that the ancestors of the forms with 

 reduced dentition possessed teeth in full numbers and that there has 

 actually been a loss of teeth in the course of descent. This conclusion 

 is abundantly confirmed by the facts of embry.ology. Take, for 

 example, the great group of the gnawing mammals or Rodentia, in 

 which the front teeth or incisors, above and below, are reduced to one 

 on each side, except in the rabbits. The incisors are chisel-shaped and 



^ Otto Maas, Die Ahstammungslchrc, pp. 273-74. 



