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, ete. 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 all 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 adaptation, 
is no less important and general than universal adaptation. 
Under this name I include a great number of very important 
