(c) — Origin of Adaptations 



The question of the origin of adaptations to their en- 

 vironment has been discussed for more than fifty years, and 

 is still unanswered. Strict followers of Darwin maintain 

 that adaptations, indeed all specific structural differences, 

 have slowly arisen as a result of the natural selection of use- 

 ful variations as adjustments to the environment. "They 

 explain the origin of adaptations on the basis of their use- 

 fulness" (Morgan). The weakness of their position lies in 

 the fact that they assume the presence of useful structures 

 without explaining how they arose. Lamarckians hold 

 that adaptations are " the accumulation of structural re- 

 sponses to the conditions of the environment". While 

 fluctuating variations do no doubt arise in this way, 

 there is no good evidence that structural and permanent 

 changes have arisen in the same way. 



The mutationists regard adaptations as the survival of 

 only those mutations sufficiently adapted to the environ- 

 ment to maintain a foothold. There is evidence that 

 many species are poorly adapted lo their surroundings, 

 and that others are over-adapted. The former will perish 

 where the struggle for existence becomes intense, and will 

 be supplanted by forms that are better adapted. Again 

 over-adapted forms can never arise by natural selection, 

 hence such forms must have originated in some other way. 



Both cases can be more readily explained by assuming 

 that new forms are constantly arising as mutations, that 

 those well adapted to their environment will likely survive 

 in the struggle for existence and leave progeny, and that 

 those not so well adapted will die but may survive for a 

 time, where the struggle is not severe. (See page 21). 



Chapter 12— JOHANxNSEN'S PURE LINES (1903) 



Johannsen's experiments with beans and barley, both 

 self:fertilizing plants, seem to show that fluctuating var- 

 iations have little or no influence on the permanent improve- 

 tnent of a race. He produced a number of Pure Lines from 

 smgle plants by self-fertilization. The members of each 

 Pure Line showed a normal variability in the weights of 

 their seeds, which differed more or less from the mean of the 

 variety. When a markedly divergent member of a par- 

 ticular line was propagated, its offspring showed regression 



■ 70 



