•sect, i.] INTRODUCTION. 5 



Without discussing this solution, we may note that it aims at 

 being a complete solution of both 



(1) The existence and persistence of differing forms, 



(2) The fact that the differing forms are adapted to 



different conditions ; 

 and (3) The causes of the Variation by which the diversity has 

 occurred. 



B. Damvin's Solution. Darwin, without suggesting causes of 

 Variation, points out that since (1) Variations occur — which they 

 are known to do — and since (2) some of the variations are in the 

 direction of adaptation and others are not— which is a necessity — 

 it will result from the conditions of the Struggle for Existence 

 that those better adapted will on tlie whole persist and the less 

 adapted will on the whole be lost. In the result, therefore, there 

 will be a diversity of forms, more or less adapted to the states 

 in which they are placed, and this is very much the observed 

 condition of living things. 



We may note that this solution does not aim at being a com- 

 plete solution like Lamarck's, for as to the causes of Variation it 

 makes no suggestion. 



The arguments by which these several solutions are supported, 

 and the difficulties which are in the way of each, are so familiar 

 that it would be unprofitable to detail them. On our present 

 knowledge the matter is talked out. Those who are satisfied with 

 either solution are likely to remain so. 



It may be remarked however that the observed cases of adap- 

 tation occurring in the way demanded on Lamarck's theory are 

 very few, and as time goes on this deficiency of facts begins to be 

 significant. Natural Selection on the other hand is obviously a 

 • true cause,' at the least. 



In the way of both solutions there is one cardinal difficulty 

 which in its most general form may be thus expressed. According 

 to both theories, specific diversity of form is consequent upon 

 diversity of environment, and diversity of environment is thus 

 the ultimate measure of diversity of specific form. Here then we 

 meet the difficulty that diverse environments often shade into 

 each other insensibly and form a continuous series, whereas the 

 Specific Forms of life which are subject to them on the whole 

 form a Discontinuous Series. The immense significance of this 

 difficulty will be made more apparent in the course of this work. 

 The difficulty is here put generally. Particular instances have 

 been repeatedly set forth. Temperature, altitude, depth of water, 

 salinity, in fact most of the elements which make up the physical 

 environment are continuous in their gradations, while, as a rule, 

 the forms of life are discontinuous 1 . Besides this, forms which 



1 It may be objected that to any organism the other organisms coexisting with 

 it are as serious a factor of the environment as the strictly physical components; 

 and that inasmuch as these coexisting organisms are discontinuous species, the 



