CLASSIFICATION AND EVOLUTION 525 



of its life, by its activity in response to the stimuli it receives 

 from its surroundings or by the dwindling of 

 The Mode of certain of its organs from lack of use, are in- 

 Lamai-ckism. herited in some degree by its offspring, and that 

 the accumulation of such small modifications pro- 

 duces at length a different kind of animal. Thus, since it is 

 known that one effect of cold upon a mammal is to increase 

 the growth of its hair, the long fur of species which live in 

 cold countries might be supposed to be due to the inherited 

 effect of the climate. Again, the effect of use upon muscles 

 is to increase their size, and in this way the great size of the 

 wing muscles of birds of strong flight might be supposed to 

 have been brought about in the course of many generations. 

 On the .other hand, the dwindling which is undoubtedly 

 caused in the organs of individuals by disuse might in 

 time bring about permanent degeneration, such, for 

 instance, as that which is found in the eyes of animals 

 which live in dark caves. This hypothesis is known as 

 the Lamarckian theory, from the name of its greatest 

 exponent. It is rejected by many zoologists at the present 

 day on account of the lack of satisfactory evidence that 

 modifications which are produced in the course of the 

 life of an individual are transmitted by it to its offspring. 

 Such modifications are known as acquired characters, and 

 over, the question of their inheritance discussion is still rife. 

 The obstacles to belief in their transmissibility lie not only 

 in the fact that numerous experiments and observations 

 have hitherto failed to prove it beyond dispute, but also in 

 the difficulty of conceiving any way in which modifications 

 in distant parts of the body can so affect the germ-cells as 

 to be handed on to the offspring. 



Another hypothesis as to ' the way in which evolution is 

 brought about is the theory of natural selection. 



Darwinism 



known as Darwinism after the great naturalist by 

 whom it was formulated. It supposes that the transformation 

 of species is caused by the destruction by adverse circum- 

 stances of certain kinds of individuals in each generation 

 before they can breed. 1 The result of this will be that the 



1 Of course the eliminated individuals would not always be killed 

 before they could breed. In some cases the reproductive period would 

 merely be cut short so that the number of offspring was lessened. 



