516 EVOLUTION 



evolved. Although Lamarck's explanation, as based upon the 

 use and disuse of organs, applies particularly to animals, he also 

 offered an explanation for evolution in plants. In case of plants, 

 in which there is no conscious effort as in animals, he assumed 

 that changes in the environment affect the body of a plant 

 directly and induce modifications which may become sufficiently 

 pronounced to characterize a new species. Since species are 

 the units of all other groupings of plants or animals, the origin 

 of new species results in the origin of new genera, new families, 

 new orders, and so on. It is, therefore, obvious that accounting 

 for the origin of species accounts for the origin of all those 

 differences upon which the various groupings of organisms are 

 based. 



Third, Lamarck believed that whatever changes a plant or 

 animal made in the form, structure, or function of its body 

 were inherited by the offspring. To the succeeding generation 

 each generation transmits what it inherited and whatever addi- 

 tional modifications it may take on. In this way modifications 

 which are only slight at first may become more pronounced in 

 succeeding generations, if the conditions remain constant, until 

 finally plants or animals so different from their ancestors as to 

 form new species may arise. He did not claim that all individ- 

 uals taking on new modifications survive but only those pos- 

 sessing changes that fit them most perfectly to their environ- 

 ment. 



Lamarck's explanation is unsatisfactory in a number of ways. 

 In the first place both observations and experiments furnish 

 much evidence that the effects of use and disuse are seldom, if 

 at all, inheritable and hence have no permanency such as the 

 characters of species have. If the effects of use and disuse are 

 not transmitted, the hypothesis that the effects of use and dis- 

 use may accumulate from generation to generation also lacks 

 support. Also, in case of plants, more recent investigations show 

 that modifications that are direct responses to the environment 

 are not generally, if at all, inheritable. In the second place his 

 explanation does not account for the desire of the animal to 

 change its habits, but simply assumes that animals change in 

 their desires, and that such changes are also transmitted. 



Darwin's Explanation. Although Charles Darwin (Fig. 44) 

 was neither the first to believe in evolution nor the first to 



