ACQUIRED CHARACTERS IN ANIMAL BREEDING 481 
modifications of bodily structure or habit which are impressed upon 
the organism in the course of. individual life. This distinction is by 
no means another instance of the hair-splitting proclivities of modern 
science; it is on the contrary a real distinction of fundamental impor- 
tance in shaping conceptions of evolution and heredity. It is un- 
necessary to give any specific examples of blastogenic characters, since 
the whole discussion of Mendelian heredity in preceding pages has been 
confined to them. Of somatogenic characters, however, it is perhaps 
well to mention a few in order to give a concrete starting point for the 
following discussion. Acquired characters include a vast number of 
characters due to environmental effects, for example, small size when 
a consequence of reduced food supply or other conditions unfavorable 
to growth, increased size consequent upon unusually favorable environ- 
mental conditions, mutilation, the effects of disease, and other modifi- 
cations of a like character. Those acquired characters which have 
their origin in response to environmental conditions have often been 
distinguished from that other class, the motive force in the develop- 
ment of which resides in the organism itself, the effects of use and 
disuse. Conspicuous examples of ‘‘achieved”’ characters as  distin- 
guished from ‘“thrust’’ characters are increases in the perfection of 
function dependent upon exercise, such as the increased speed of the 
trained race horse and the increased sharpness of intellect of the 
trained mind. 
As Thomson has stated it, the precise question at issue is this: Can 
a structural change in the body, induced by some change in use 
or disuse, or by a change in surrounding influence, affect the germ cells 
in such a specific or representative way that the offspring will through its 
inheritance exhibit, even in a slight degree, the modification which the 
parent acquired? 
Obviously a problem such as this must require very critical treat- 
ment, and much, if not all of the evidence brought forward in support 
of the belief in the inheritance of acquired characters suffers from failure 
to fulfil the requirements of a rigid proof. Thomson has given an excel- 
lent extended treatment of this side of the case, as well as of the subject 
of acquired characters in general. 
To satisfy the rigid requirements of an experimental proof any evi- 
dence of the inheritance of acquired characters must fulfil the following 
conditions: 
First, a specific character or modification in the soma must be im- 
pressed upon the organism by a known factor in its environment or in 
its exercise of bodily function. 
Second, the character or modification should be new. There must be 
no question of the reappearance of ancestral traits or characters, or of 
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