XV THE PHILOSOPHY OP ZOOLOGY 649 



The first noteworthy attempt to solve the problem regarding the 

 nature of the forces by means of which evolution has taken pkce 

 was made, long before evolution was generally accepted among 

 men of science, by Lamarck in his Philosophie Zoologique, published 

 in 1809. Lamarck's view was that evolution of new forms has 

 taken — and is taking — place, in great measure owing to the direct 

 action of the conditions of life on the organism, but still more 

 owing to the use and disuse of organs. The surroundings or 

 environment of the animal or plant produce a direct effect on the 

 individual — bring about slight modifications in one direction or 

 another, and these slight differences are transmitted by inherit- 

 ance to the next generation — such slight modifications going on, 

 generation after generation, producing eventually a marked effect 

 on the characters of the organism. The chief agencies that might 

 be supposed to act in this way are climate, the nature of the 

 country, and food. But, in addition to these, Lamarck attributes 

 considerable influence to the use and disuse of organs. The 

 exercise of a part tends to increase its size and efficiency, and 

 such increase may be and frequently is, according to Lamarck, 

 transmitted to the succeeding generation. In this way, in the 

 course of a number of generations, very great changes might 

 be brought about. To take an example which is often quoted, 

 Lamarck accounts for the great length of the neck of the Giraffe 

 as compared with other Euminants by the supposition that it has 

 been brought about by continuous efforts made by the animals 

 through a long series of generations to reach higher and higher 

 among the foliage of the trees from which they derive their main 

 subsistence. Similarly, the disuse of a part, in Lamarck's view, 

 gradually leads to its diminution, and perhaps ultimately to its 

 complete disappearance. In this way he would explain the dis- 

 appearance of the hind-limbs in the Cetacea, of both pairs of 

 limbs in the Snakes, of the olfactory nerves in aquatic Mammals, 

 and so on. Whether differences which are produced in the in- 

 dividual organism by surrounding conditions or by its own efforts 

 may be transmitted by inheritance to succeeding generations is not 

 yet a settled point : we shall have again to refer to this question 

 — the question of the inheritance of acquired characters — at a later 

 stage. That such inheritance, if it takes place, could account for 

 the development of all the various groups of animals and plants 

 is not held by many biologists at the present time. 



Darwinian Theory. — It is to Charles Darwin that we owe 

 the most thorough and consistent explanation of evolution that 

 has hitherto been put forward — the explanation known as the 

 theory of Natural Selection. The development of this theory and 

 the share taken in it by Wallace will be sketched in the historical 

 section. The two main supports of Darwin's theory are two sets of 

 biological phenomena known respectively as the struggle for existence 



