xv THE PHILOSOPHY OF ZOOLOGY 613 



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 Ruminants by the supposition that it has 

 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 veiw, 

 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 

 are 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 this theory are two sets of 

 biological phenomena known respectively as the struggle for exist- 

 ence 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 con- 

 ditions. The plant must find a place with soil containing certain 

 constituents, a certain degree of moisture and of sunlight. For 

 spots presenting the necessary favourable conditions there is 

 constantly going on a competition between individual plants of 

 one species and between the members of different species. The 

 nature of this struggle is well seen when a piece of garden ground 

 is allowed to run to waste. Its surface is soon overgrown by 

 weeds of a variety of kinds, which kill out some of the original 

 garden plants. Byandby the more hardy weeds kill out and 

 replace such weaker forms as may first have obtained a footing, 

 till an entirely new set of weeds may take the place of those that 

 first appeared. Again, it was shown by Darwin that in turf which 



