EVOLUTION OF THE COLORS OF BIRDS. 3 



cess became more animated and energetic through the 

 increased proportion of oxygen in the atmosphere. Thus 

 there arose a higher temperature of the blood, an in- 

 creased activity of the nerves and muscles, and the scales 

 of the reptiles became the feathers of birds," etc. 



The principle of use and disuse, and the transmission 

 of acquired characters, are very closely related and stand 

 or fall together. Darwin, in his Origin of Species, and 

 throughout all his subsequent writings, accepted them 

 both as supplementary to his own doctrine of natural selec- 

 tion, in accounting for the origin of species. It was, of 

 course, generally admitted that an individual could, dur- 

 ing the course of its life, acquire characters peculiar to 

 itself, or could modify its parts by use or disuse. More- 

 over, it was commonly held that such modifications could 

 be transmitted by the individual to its offspring. To 

 this view Darwin was committed, and in order to ex- 

 plain it in a rational way he formulated the provisional 

 hypothesis of pangenesis, which he describes in the fol- 

 lowing language: " This important distinction between 

 transmission and development will be best kept in mind 

 by the aid of the hypothesis of pangenesis. According 

 to this hypothesis, every unit or cell of the body throws 

 off gemmules or undeveloped atoms, which are trans- 

 mitted to the offspring of both sexes, and are multiplied 

 by self division. They may remain undeveloped dur- 

 ing the early years of life or during successive genera- 

 tions; and their development into units or cells, like 

 those from which they were derived, depends on their 

 •afTmity for, and union with other units or cells pre- 

 viously developed in the due order of growth."* 



Mr. Eomanes has presented Darwin's hypothesis of 

 pangenesis! in so able and comprehensive a manner 



*Descent of Man, p. 228. 



tWeismann's Theory of Heredity by George J. Bomanes. Contempo- 

 rary Eeview. May 1890, pp. 686-699. 



