138 COMPARATIVE PHYSIOLOGY. 



not in reality the same in all respects as when its existence be- 

 gan, though we may be quite unable to detect the changes ; and 

 when two infusorians conjugate, the one brings to the other 

 protoplasm different in molecular behavior, of necessity, from 

 having had different experiences. We attach great importance 

 to these principles, as they seem to us to lie at the root of the 

 whole matter. What has been said of these lower but inde- 

 pendent forms of life applies to the higher. All organisms are 

 made up of cells or aggregations of cells and their products. 

 For the present we may disregard the latter. When a muscle- 

 cell by division gives rise to a new cell, the latter is not identi- 

 cally the same in every particular as the parent cell was origi- 

 nally. It is what its parent has become by virtue of those ex- 

 periences it has had as a muscle-cell per se, and as a member of 

 a populous biological community, of the complexities of which 

 we can scarcely conceive. 



Now, as a body at rest may remain so, or may move in a 

 certain direction according to the forces acting upon it exactly 

 counterbalance one another, or produce a resultant effect in the 

 direction in which the body moves, so in the case of heredity, 

 whether a certain quality in the parent appears in the offspring, 

 depends on whether this quality is neutralized, augmented, or 

 otherwise modified by any corresponding quality in the other 

 parent, or by some opposite quality, taken in connection with 

 the direct influence of the environment during development. 



This assumption explains among other things why acquired 

 peculiarities (the results of accident, habit, etc.) may or may 

 not be inherited. 



These are not usually inherited because, as is to be expected, 

 those forces of the organism which have been gathering head 

 for ages are naturally not easily turned aside. Again, we urge, 

 heredity must be more pronounced than variation. 



The ovum and sperm-cell, like all other cells of the body, 

 are microcosms representing the whole to a certain extent in 

 themselves — that is to say, cell A is what it is by reason of what 

 all the other millions of its fellows in the biological republic 

 are ; so that it is possible to understand why sexual cells repre- 

 sent, embody, and repeat' the whole biological story, though it 

 is not yet possible to indicate exactly how they more than others 

 have this power. This falls under the laws of specialization 

 and the physiological division of labor ; but along what paths 

 they have reached this we can not determine. 



