The Nervous System. 



183 



circulation and secretion, brings about the needed modification. In 

 another habitat, where such weapons of defense are not wanted, they 

 are not developed. If an additional phalanx be serviceable in the 

 digit of a loon to give expanse to an inter-digital web, the extra bone 

 is forthcoming. The transformation does not occur in the growth of 

 the individual, but is begun in embryonic states where tendency to 

 modify is greatest. Probabl} 7 several generations are passed in making 

 an appreciable modification. The two-toecl sloth did not lose three 

 digits in less than a hundred generations, though a saltus or leap is 

 possible at any time. It can not be rationally conjectured how many 

 generations it would require to establish two large toes in the African 

 ostrich. Possibly it was a lusus naturce at first, and the stronger 

 state became permanent and prevailing, old forms being less desirable 

 in sandy countries, at length died out. However, it is likety that the 

 nerves distributed to a chick of four toes, failed gradually in the 

 energy of two inner toes, and correspondingly gained in the two outer 

 digits. In the hind foot of a kangaroo, the dwarfing process in inner 

 toes is at present visible or appreciable. If kangaroos could be kept 

 on the sands of Sahara for ten generations, it might be expected that 

 the diminutive and insignificant digits on the inside of the hind feet 

 would entirely disappear. It is easy to see that the flippers of pen- 

 guins have either degenerated from wings, or have been evolved from 

 fins. The modified form was inspired by function ; and the latter 

 leaned towards the physical transformation which would be Vjeneficial 

 to the race. 



The outer gilled axolotl or siredon of New Mexico, under some circum- 

 stances becomes transformed into an inner gilled amblystoma. Plenty 

 of food and increased light and heat favor the marked modification. 

 Nervous energies must have much to do with the changes occurring in 

 the larval, crysalis, and butterfty states of an insect. The worm has 

 as many ganglia as it has pairs of legs, to say nothing of the group 

 that presides over mastication and deglutition. In the winged and 

 mature states, the ganglia are lessened in number, and greatly modi- 

 fied in function. It is not reasonable or logical to say that neural 

 ganglia either make or modify themselves, but they embrace a vital 

 attribute, which under the influence of a peculiar environment, leads 

 to transformation. A nerve alone can not make or unmake a digit or 

 phalanx ; yet a preponderating influence brought to bear upon some 

 of its filaments, cells, or tubules, could secure a change. 



While nature seems to adhere to well known forms, " she" quite 

 frequently manifests a disposition to take departures. This tendency 



