178 THE DOCTRINE OF DESCENT. 
behind or by the side of one another; hence on the 
division of labour. This Haeckel has designated 
divergent adaptation. It gives rise to the remark- 
able polymorphism, which appears especially in the 
marvellous forms of the Hydra tuba; and, higher up, in 
the segmentation of the classes of the Termites and 
the Bee, &c. 
So far as modification coincides with adaptation, the 
direct adaptations hitherto discussed may be opposed 
by a series of indirect adaptations. Among these may 
be comprised a series of phenomena of which the causes 
do not fall within the life of the individual, but are to 
be sought in influences by which the parents were 
affected. It is obvious that we here come into contact 
with the province of heredity in a manner well known 
to breeders. Thus H. v. Nathusius, in his studies on 
the formation of the pig’s skull," says:—“ From the 
facts here collected, it is plain that the transmission, the 
transfer of the form of head from the parents to the 
offspring, does not unconditionally ensue. If the form 
of skull, which we will briefly term the cultivated form, 
be the product of nutrition and mode of life, hence of 
external influence,—if it can be differently formed in the 
same individual, and is therefore not constant,—in that 
case the heredity of this form cannot be spoken of 
without qualification. The form itself will not be trans- 
mitted to the offspring, but only the tendency to the 
form. This may be inferred from the circumstance 
that from generation to generation, and to a certain 
degree, the form increases in peculiarity. If we reara 
common with a thoroughbred pig, and if we allow 
exactly the same influences of nutriment and keeping 
