550 - Heredity and Evolution 



proving any homozygous stock as a result of 

 directive influences from the environment. 

 The environment, to put it more technically, 

 is able to effect a limited improvement in die 

 developmental performance of the group of 

 genes determining the root system of the 

 stock, but the environment has not been able 

 to produce any measurable change in the 

 intrinsic nature of these genes. 



Similarly a great many experiments have 

 been performed on other organisms, using a 

 variety of adverse environmental conditions, 

 such as chronic alcoholism. One inbred stock 

 of white rats was daily subjected to extreme 

 alcoholism from the time of weaning until 

 death, for more than ten generations. Dur- 

 ing this period of extreme debauchery, there 

 were among the litters a very high propor- 

 tion of feeble, stunted individuals, but the 

 offspring ol all such rats were restored to a 

 full measure of growth and vigor as soon as 

 they were taken off the alcoholic treatment. 

 The alcohol did not alter the genie complex 

 of the treated animals, and consequently 

 none of the adverse Acquired characteristics 

 found a permanent place in the heritable 

 constitution of the race. 



Lamarck's theory laid great stress on the 

 question of use and disuse of the various 

 organs of the species. The horse, for example, 

 was considered to have arisen from a slower 

 running form that inhabited the open plains, 

 where a constant exercise of speed was essen- 

 tial in the avoidance of carnivorous enemies. 

 Accordingly, improvements in the skeletal 

 and muscular structures of the legs were 

 thought to have occurred gradually as a 

 result of the excessive use of these organs 

 demanded by the environmental conditions. 

 And conversely, blind species, which are 

 found inhabiting deep, dark caves, were 

 thought to have arisen as a result of a gradual 

 degeneration of the eyes, due to many gener- 

 ations of disuse. 



Two main fallacies of the use and disuse 

 theory are: (1) all experiments designed to 

 prove an inheritance of environmentally in- 

 duced changes in somatic structures have 



been unsuccessful; and (2) although certain 

 factors in the environment, such as heat and 

 short-wave radiations, may accelerate the 

 appearance of mutations, these heritable 

 variations are random and are not directed 

 by the environment toward a greater fitness 

 of the variant with reference to its habitat. 

 One such experiment involved the rearing 

 of fruit flies in total darkness for more than 

 60 generations — an equivalent of many cen- 

 turies in human history. When this experi- 

 ment was finished and the Hies at long last 

 were restored to light, no changes coidd be 

 found, either in the structure of eyes or in 

 the responses of the flies to light. Similarly, 

 the old custom of binding the feet of high- 

 caste Chinese women — -which made it virtu- 

 ally impossible tor these individuals to walk 

 — has had no heritable effect upon the foot 

 dimensions of the race, even after many cen- 

 turies. Likewise, many sorts of mutilation, in- 

 cluding the surgical removal of certain 

 organs, have been adhered to religiously by 

 various peoples since the dawn of human 

 societies — without leaving the slightest mark 

 upon the genetic constitution of any race. 

 These environmentally induced modifications 

 do not penetrate to the level of the germ cells 

 of the species; and no change can play a sig- 

 nificant role in evolution unless it involves 

 the genes of the germ cells. Thus it can be 

 said that the environment has no directive 

 influence upon germ cell mutations, except 

 that the environment eliminates such muta- 

 tions as may decrease and selects such muta- 

 tions as may increase the general fitness of 

 the species. 



The impact of modern genetics upon the 

 theory of natural selection has thus been to 

 clarify and strengthen it, without essentially 

 modifying this theory as developed by Dar- 

 win. The known laws of variations and 

 heredity operate by providing material that 

 is sifted and screened by natural selection. 

 These somewhat haphazard processes logi- 

 cally account for the origin of the species of 

 animals and plants that exist today; and sci- 

 ence can see no other alternative. 



