TRANSMISSION OF MODIFICATIONS 



351 



giving the appearance of having been specially created for their 

 particular surroundings. We know enough of the causes of 

 variation, however, to realize that this " fit " between the animal 

 or the plant and its environment has been brought about by the 

 direct action of outside conditions upon organisms somewhat 

 plastic and capable of becoming adapted to a wide range of 

 circumstances, all individuals not possessing this adaptability 

 having long since disappeared by natural selection. 



Nothing is more simple and natural than to assume that 

 when the individual has acquired some modification through 

 the influence of the environment it will transmit this modifica- 

 tion to its descendants, and that what was at first impressed 

 from without gradually becomes hereditary, exhibiting all the 

 cumulative effect of transmissible qualities. All appearances 

 are in favor of such an assumption, and it is the simplest and 

 most direct explanation of the phenomena of adaptation and of 

 the well-known harmony that always exists between a species 

 and its environment. 



But there are at least two other methods by which the envi- 

 ronment impresses itself strongly upon the species, both of 

 which are always at work, both of which achieve large results 

 of an exactly similar character and appearance, and which must 

 therefore be either subtracted or fully accounted for before we 

 are warranted in assuming results due to direct transmission. 



The first of these is the direct effect of the conditions of life 

 acting separately upon all the individuals of each generation 

 and all in the same direction. This has all the appearance of 

 transmission, but it is not cumulative, and can bring about no 

 better adaptation in the species, and no greater total change, 

 than can be wrought for each individual during its lifetime. 

 The other is due to the selective effect of the environment, 

 which is cumulative, and, so far as we can see, abundantly able 

 to account for all the phenomena of adaptation and for the full 

 effects of environment. 



The environment always selective. Some individuals of every 

 generation fail utterly to endure the conditions of life and, thus 

 failing, they leave no descendants. The next generation, there- 

 fore, being descended from the more adaptable individuals, is 



