30 BIOLOGICAL LECTURES. 



greatest weakness, and in it, no less than in all preceding 

 hypotheses, the theory of a separate category of particles car- 

 rying hereditary potentialities again appears. 



The one criticism that holds of all these hypotheses is that 

 they are one-sided and ignore a most important set of factors 

 in inheritance, namely, the purely statical ones, or those aris- 

 ing from the mere physical properties of the living matter of 

 the germ viewed as if it were a dead, inert mass, subject to 

 the operation of the reciprocal attraction for one another of its 

 constituent particles. All of these hypotheses, moreover, as- 

 sume that it is only some of the matter of the germ that is 

 concerned in the process of hereditary transmission, and that 

 the remainder may be regarded as passive. The entire germ, 

 on the contrary, or all of it that undergoes development, must 

 be considered as a single whole, made up of a vast number of 

 molecules built up into a mechanism. Such a molecular mech- 

 anism, it must be supposed, cannot set free the potential energy 

 of its parts except in a certain determinate order and way, 

 within certain limits, in virtue of the initial physical structure 

 of the whole. If the germ is free to do that, as must happen 

 under proper conditions, as a mechanism, its parts, as they arc 

 thus formed by their own metabolism, it may be assumed, will 

 inevitably and nearly recapitulate the ancestral development or 

 that typical of the species. It must do this as a mere dynam- 

 ical system or mechanism, the condition of which at one phase 

 determines that of the next, and so on, to the completion of 

 development. 



In the present state of our knowledge we are not prepared 

 to frame a purely mechanical hypothesis of inheritance that 

 shall answer every requirement, in spite of the fact that no 

 other is possible. Herbert Spencer and Professor Haeckel 

 long ago pointed out that such an hypothesis is a necessity, 

 growing out of the very requirements that must be satisfied in 

 any attempt to coordinate the phenomena of biology with those 

 of the not-living world. The material basis of life is always a 

 chemically and mechanically compounded substance. To the 

 very last molecule, such a body must betray evidence of 

 arrangement or structure of its parts that should make it a 



