IX THE HISTORY AND SCOPE OF ZOOLOGY 371 



the causes of variation in animals and plants, the 

 laws of the transmission of parental characters, the 

 share of each parent in the production of the char- 

 acters of the offspring, atavism, and the relations of 

 young to parents as to number, sex, nourishment, and 

 protection. 



An important development of Darwin's conclu- 

 sions is actually in progress and deserves special 

 notice here, as it is the most distinct advance in the 

 department of Bionomics since Darwin's own writings, 

 and at the same time touches questions of funda- 

 mental interest. The matter strictly relates to the 

 consideration of the " causes of variation," and is as 

 follows. 



The fact of variation is a familiar one. No two 

 animals, even of the same brood, are alike : whilst 

 exhibiting a close similarity to their parents, they yet 

 present differences, sometimes very marked differences, 

 from their parents and from one another. Lamarck 

 had put forward the hypothesis that structural altera- 

 tions acquired by a parent in the course of its life are 

 transmitted to the offspring, and that, as these struc- 

 tural alterations are acquired by an animal or plant 

 in consequence of the direct action of the environ- 

 ment, the offspring inheriting them would as a con- 

 sequence not unfrequently start with a greater fitness 

 for those conditions than its parents started with. In 

 its turn, being operated upon by the conditions of 

 life, it would acquire a greater development of the 

 same modification, which it would in turn transmit to 

 its offspring. In the course of several generations, 



