xv THE PHILOSOPHY OF ZOOLOGY 629 



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

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

 taken and is taking place, ift 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 inheritance 

 to the next generation such slight modifications going on, genera- 

 tion 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 con- 

 siderable 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 

 Ruminants 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 disappearance 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 individual organism by surrounding conditions 

 or by its own efforts may be transmitted by inheritance to suc- 

 ceeding 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 

 and variation, both of which have to be understood before it is 

 possible to grasp the theory of natural selection. 



Struggle for Existence. In order that it may flourish, there 

 are necessary for every species of plant and animal certain conditions. 



