108 GENERAL ZOOLOGY 



mole-cricket and mantids, the enlarged hind legs of the locust, 

 and the absence of eyes in certain cave-inhabiting insects. 

 With this principle are associated the names of Erasmus 

 Darwin, grandfather of Charles Darwin, and the French 

 naturalist Lamarck (see p. 448). 



The chief difficulty in the application of the principle of the 

 inheritance of acquired characters as a factor in evolution lies 

 in the fact that we have little or no evidence that the charac- 

 ters acquired by use or disuse during the life of an individual 

 are transmitted to its descendants, though some effort has 

 been made of late years to furnish this direct evidence. The 

 most important experiments along this line are those of Brown- 

 Sequard, a Franco- American physiologist (18191894). Pie 

 succeeded in producing epilepsy in guinea-pigs born of parents 

 which had been rendered epileptic by an injury to the spinal 

 cord. Exophthalmia (a disease characterized by abnormal 

 protrusion of the eyeball) was also transmitted through sev- 

 eral generations. Certain mutilations, produced by the ani- 

 mals eating off their hind-leg toes after the latter have been 

 rendered insensible to pain by cutting the nerve leading to 

 them, also seem in some cases to have been transmitted to the 

 descendants. The objection is made to these experiments 

 that they may show the result of a transmitted disease rather 

 than the inheritance of an acquired character. 



The Direct Influence of the Environment. In connection 

 with the principle of the inheritance of characters acquired 

 through use or disuse, Lamarck held that the changes in the 

 environment directly brought about changes in the organism, 

 and that these changes were transmitted, to the descendants 

 (see p. 449). This and the preceding principle are therefore 

 often spoken of as Lamarckian factors in evolution, while 

 natural and sexual selection are termed Darwinian factors. 



All organisms have to exist under certain conditions of 

 pressure either of liquids or gases; they are adapted to a 



