DIRECT ADAPTATION. 233 



adapted itself to different conditions of life. The original 

 difference of the individual processes of development, evi- 

 dently becomes greater the longer the life lasts and the 

 more various the external conditions which influence the 

 separate individuals. This may be demonstrated in the 

 simplest manner in man, as well as in domestic animals and 

 cultivated plants, in which the vital conditions may be ar- 

 bitrarily modified. Two brothers, of whom one is brought 

 up as a workman and the other as a priest, develop quite 

 differently in body as well as in mind ; in like manner, two 

 dogs of one and the same birth, of which one is trained as a 

 sporting dog and the other chained up as a watch dog. The 

 same observation may also readily be made as to organic in- 

 dividuals in a natural state. If, for instance, one carefully 

 compares all the trees in a fir or beech forest, which con- 

 sists of trees of a single species, one finds that among 

 all the hundreds or thousands of trees, there are not two 

 individual trees completely agreeing in size of trunk and 

 other parts, in the number of branches, leaves, etc. Every- 

 where we find individual inequalities which, in part at 

 least, are merely the consequences of the different conditions 

 of life under which the trees have developed. It is true we 

 can never say with certainty how much of this dissimilarity 

 in aU the individuals of every species may have originally 

 been caused by indirect individual adaptation, and how 

 much of it acquired under the influence of direct or uni- 

 versal adaptation. 



A second series of phenomena of direct adaptation, which 

 we may comprise under the law of cumulative adaptoMon, 

 is no less important and general than universal adaptation. 

 Under this name I include a great number of very important 



