THE ORIGIN AND HISTORY OF ANIMAL LIFE 



621 



daily Mendel and de Vries. Buffon (1707- 

 1788) was a French writer on natural history 

 who published evidence, such as we use 

 today, to prove that species evolve from 

 other species; he apparently believed that 

 this was the result of environmental modifi- 

 cations that were inherited. He was followed 

 by Erasmus Darwin ( 1 731-1 802 ) , a grand- 

 father of Charles Darwin, who, in both 

 poetrv and prose, expressed a belief in or- 

 ganic e\'olution and in the hypothesis that 

 changes take place as the result of the in- 

 heritance of environmental modifications. 



Inheritance of acquired 

 characters 



Lamarck (1744-1829) was principally re- 

 sponsible for the belief, which prevailed dur- 

 ing much of the nineteenth century, in the 

 inheritance of acquired characters. Accord- 

 ing to the Lamarckian hypothesis of organic 

 evolution, modifications due to use and dis- 

 use, or to direct action of environmental 

 factors, are transmitted to the offspring; that 

 is, traits acquired during an animal's life- 

 time are inherited by that animal's offspring. 

 Such inherited modifications would account 

 for the ability of antelope to escape their 

 enemies by running rapidly, since the speed 

 acquired by one generation would be passed 

 on to the next, and so ad infinitum. Simi- 

 larly, the giraffe would have acquired its 

 long neck as a result of its habit of browsing, 

 generation after generation, on the leaves of 

 trees. These are examples of the supposed 

 effects of use. If organs were not used, they 

 were supposed to degenerate, as for example, 

 the eyes of animals that live in caves. 



Many investigators have attempted to ob- 

 tain experimental evidence of inheritance of 

 acquired traits, which must be proved be- 

 fore the Lamarckian hypothesis can be 

 accepted. No evidence has yet been pre- 

 sented that is acceptable to most zoologists. 

 Offspring do not appear to inherit modifica- 

 tions unless they are due to changes in the 

 genes. The speed of the antelope, the long 

 neck of the giraffe, and the degenerate eyes 



of cave animals are correlated with the dif- 

 ferent environments in which these types of 

 animals live and are best explained as the 

 results of genetic change. Thus, the environ- 

 ment indirectly affects the genetic makeup 

 of a species by operation of natural selection, 

 which retains those genetic combinations 

 best suited to prevailing conditions and dis- 

 cards those which are not fitted to them. 

 The conclusion is that Lamarck's hypothesis 

 is not valid. 



Mutations in relation 

 to evolution 



De Vries (1848-1935) was a Dutch 

 botanist whose principal studies were carried 

 on with evening primroses. He observed sud- 

 den large changes in his primroses that were 

 inherited and which he considered to be 

 mutations. After 20 years of study, the same 

 length of time devoted to the study of 

 natural selection by Darwin, he published a 

 book on mutations. His studies focused the 

 attention of biologists on mutations, which 

 have subsequently found to be of general oc- 

 currence. 



Mutations were known to earlier breeders. 

 Among these may be mentioned the short- 

 legged sheep which appeared in a flock in 

 Massachusetts in 1791 and from which was 

 developed the Ancon breed of sheep. This 

 mutation was of value to the farmer because 

 these sheep could not jump over the low 

 stone fences of New England. This breed 

 became extinct about 90 years ago, but some 

 50 years later, a short-legged lamb appeared 

 in the flock of a Norwegian farmer. From 

 this, a new strain of Ancon sheep has been 

 bred (Fig. 439). 



Mutations have appeared frequently in 

 animals bred for purposes of study, the most 

 famous being those of the fruit fly, Dro- 

 sophila melanogaster (Fig. 440). Investiga- 

 tors have examined millions and millions of 

 flies belonging to this species and have de- 

 scribed many different mutations in them. 



As already noted, some mutations are due 

 to changes in the genes which are respon- 



