146 Animal Life and Intelligence. 



"body-cells produce no change on the germ-plasm which they 

 may contain. We regarded this hypothesis as a retrograde 

 step, much as we admire the genius of its propounder, and 

 considered that the fiction of two protoplasms, distinct and 

 yet commingled, is little calculated to advance our com- 

 prehension of organic processes. 



In the known and observed phenomena of cellular con- 

 tinuity and cell-differentiation, we found a sufficiently satis- 

 factory hypothesis of heredity. The reproductive cells 

 are the outcome of normal cell-division, and have been 

 differentiated and set apart for the special work of develop- 

 mental reproduction, as others have been differentiated 

 and set apart for other protoplasmic functions. Such a 

 view adequately accounts for hereditary continuity, for 

 there is a continuity of the germinal cells, the bearers of 

 heredity. But how, we repeat, on this view or any other 

 hypothesis of direct continuity, are the origin of variations 

 and then transmission to be accounted for ? 



Every individual organism reacts more or less markedly 

 under the stress of environing conditions. The reaction 

 may take the form of passive resistance, or it may be 

 exemplified in the performance of specially directed motor- 

 activities. The power to react in these ways is inborn ; 

 but the degree to which the power is exercised depends 

 upon the conditions of existence, and during the life of the 

 individual the power may be increased or diminished accord- 

 ing to whether the conditions of life have led to its exercise 

 or not. The effects of training and exercise on the per- 

 formance of muscular feats and in the employment of mental 

 faculties are too well known to need special exemplification. 

 By manual labour the skin of the hand is thickened ; and 

 by long-continued handling of a rifle a bony growth caused 

 by the weapon in drilling, the so-called exercierknochen of 

 the Germans, is developed. Now, it is clear that if these 

 acquired structures or faculties are transmitted from 

 parent to offspring, we have here a most important source 

 and origin of variations — a source from which spring varia- 



