OTHER THEORIES OF SPECIES-FORMING. 267 



to biology is his generally sound distinction between the 

 germ-plasm and the soma-plasm and parts of the many- 

 celled body. At maturity the animal body is composed of a 

 small mass of germ-plasm (germ-cells) situated in the 

 ovaries or testes, and a great mass of somatic tissues and 

 organs, all the rest of the body, in fact. Now what is the 

 condition that exists in the body after a somatic part is 

 modified by use or disuse or by other functional stimulus, as 

 when a muscle is enlarged by exercise, the sole of the foot 

 calloused by going barefoot, an ear more finely attuned by 

 training? We have a definite physical change in a definite 

 organ, but is the germ-plasm in any way changed or 

 affected by this superficial or specific somatic modification, 

 or if changed is it changed so that it will reproduce in its 

 future development a similar change in the same organ of 

 the future new individual ? What possible mechanism have 

 we in the body to produce or insure such an effect on the 

 germ-plasm ? The answer is obvious and flat ; we certainly 

 know of no such mechanism ; in fact what we do know of 

 the relation of the germ-cells to the rest of the body makes 

 any satisfactory conception of such a mechanism as yet 

 impossible. 



Not that certain external conditions may not directly 

 affect the germ-cells, imbedded and concealed as they are 



in the body. Varying conditions of tempera- 

 Difficulties in J * 



the way of ture,* of humidity, and of magnetism, perhaps, 



hhwitencfof certainl y anything influencing the food supply 

 acquired char- and nutrition, can influence the germ-cells at 

 acters. n t h e rest of the 



body. But will this influence photograph on these un- 

 differentiated cells the same picture that it impresses on the 



* While temperature may be looked on as an extrinsic influence 

 affecting germ-cells as well as all other parts of the body, it must be 

 kept in mind that warm-blooded animals (birds and mammals) 

 regulate the inner temperature of the body. So changes in external 

 temperature would but slightly, or not at all, affect the germ-cells. 



