350 GENERAL ZOOLOGY 



selection must work to produce new types. These and 

 others that might be cited, however, in no way invalidate 

 the theory as a whole, and it is looked upon by the majority 

 of scientific men as the best working hypothesis we have. 



The following concrete illustration may serve to compare 

 the differences between the two theories. From palseon- 

 tological evidence it is known that the modern horse de- 

 scended from ancestors which had several toes on each foot. 

 According to the Lamarckian hypothesis the one-toed con- 

 dition probably came about through use and disuse. In 

 order to run more swiftly horses stood on the tips of the 

 toes; the middle digit was used most and hence grew larger 

 through successive generations; the toes on either side 

 dwindled away with disuse. A Darwinian explanation of 

 the same facts would suppose that ancestral horses varied 

 among themselves; that those with larger middle toes were 

 able to run faster and hence survived more often in the 

 struggle for existence; that the middle toes were progress- 

 ively larger and the side toes smaller because such condi- 

 tions better fitted the environment. According to the 

 first explanation the habit of using the middle toe and 

 neglecting those on the side led to a one- toed condition; 

 according to the second, the same end was attained by the 

 selection by the environment of variations which made 

 horses better fitted to survive in the struggle for existence. 



VARIATION AND HEREDITY 



Since the scientific world accepted the facts of evolution 

 and had theories as to how the transformations came about, 

 attention has naturally been directed to discovering how ani- 

 mals actually do vary and just how characteristics are trans- 

 mitted from one generation to another. Variations have 

 been carefully studied by statistical methods; thousands 

 of individual animals have been measured and compared. 

 Tests have also been made by breeding experiments to 

 determine how particular characteristics are transmitted 



