BIOLOGICAL EVOLUTION 6 1 



Darwin further grants that multitudes of eggs and organisms 

 are destroyed by accidental causes in which case the quality of the 

 stock would not be affected.^ 



Variation and Survival. — The theory of natural selection was 

 suggested and built up to a considerable extent on principles long 

 observed in artificial breeding. The fact of considerable variation 

 in each new generation had been observed for thousands of years, 

 and use made of the principle of " selection " to secure desirable 

 traits. Darwin accepted the well-known facts of variation with- 

 out attempting to explain their efficient cause or causes. He 

 recognized two kinds, the indefinite and the definite. 



The direct action of changed conditions leads to definite or indefinite 

 results. In the latter case, the organism seems to become plastic, and we 

 have much fluctuating variability. In the former case the nature of the 

 organism is such that it yields readily, when subjected to certain conditions, 

 and all, or neariy all, the individuals become modified in the same way. 



It is very difficult to decide how far changed conditions, such as of climate, 

 food, etc., have acted in a definite manner. There is reason to believe that 

 in the course of time the effects have been greater than can be proved by 

 clear evidence. . . . When a variation is of the slightest use to any being, 

 we cannot tell how much to attribute to the accumulative action of natural 

 selection, and how much to the definite action of the conditions of life.^ 



In response to an objection that a single variation would soon be 

 swamped, our author admits the force of the point and suggests 

 an explanation that is very close to the theory of " organic 

 selection " formulated later by Lloyd Morgan, Osborn and 

 Baldwin. " It should not ... be overlooked," he says, " that 

 certain rather strongly marked variations, which no one would 

 rank as mere individual differences, frequently recur owing to a 

 similar organization being similarly acted on, — of which fact 

 numerous instances could be given with our domestic productions. 

 In such cases, if the varying individual did not actually transmit 

 to its offspring its newly acquired character, it would undoubtedly 

 transmit to them, as long as the existing conditions remained the 

 same, a still stronger tendency to vary in the same manner." ' 

 T. H. Morgan thinks this and what immediately follows invali- 

 dates much that had been claimed previously for natural 



* Origin of Species, p. 68. ' Ibid., p. io6. Cf. p. 170. ' Ibid., p. 72. 



