ALLEGED SPONTANEOUS GENERATION, ETC. 705 



phenomena of Genesis, Heredity, and Variation. In one respect 

 only am I conscious of having so inadequately explained myself, 

 as to give occasion for a misinterpretation the one made by the 

 Westminster reviewer above referred to. By him, as by your own 

 critic, it is alleged that in the idea of " inherent tendencies " I have 

 introduced, under a disguise, the conception of " the archaeus, vital 

 principle, nisus formativus, and so on." This allegation is in part 

 answered by the foregoing explanation. That which I have here 

 to add, and did not adequately explain in the Principles of Biology, 

 is that the proclivity of units of each order towards the specific 

 arrangement seen in the organism they form, is not to be under- 

 stood as resulting from their own structures and actions only ; 

 but as the product of these and the environing forces to which 

 they are exposed. Organic evolution takes place only on condi- 

 tion that the masses of protoplasm formed of the physiological 

 units, and of the assimilable materials out of which others like 

 themselves are to be multiplied, are subject to heat of a given 

 degree are subject, that is, to the unceasing impacts of undula- 

 tions of a certain strength and period ; and, within limits, the 

 rapidity with which the physiological units pass from their indefi- 

 nite arrangement to the definite arrangement they presently assume, 

 is proportionate to the strengths of the ethereal undulations fall- 

 ing upon them. In its complete form, then, the conception is 

 that these specific molecules, having the immense complexity above 

 described, and having correspondent^ complex polarities which 

 cannot be mutually balanced by any simple form of aggregation, 

 have, for the form of aggregation in which all their forces are 

 equilibrated, the structure of the adult organism to which they 

 belong ; and that they are compelled to fall into this structure by 

 the co-operation of the environing forces acting on them, and the 

 forces they exercise on one another the environing forces being 

 the source of the power which effects the re-arrangement, and 

 the polarities of the molecules determining the direction in which 

 that power is turned. Into this conception there enters no trace 

 of the hypothesis of an " archaeus or vital principle ; " and the 

 principles of molecular physics fully justify it. 



It is, however, objected that " the living body in its develop- 

 ment presents a long succession of differing forms ; a continued 

 series of changes for the whole length of which, according to Mr. 

 Spencer's hypothesis, the physiological units must have an ' in- 

 herent tendency.' Could we more truly say of anything, ' it is 

 unrepresentable in thought ? ' " I reply that if there is taken into 

 account an element here overlooked, the process will not be found 

 " unrepresentable in thought." This is the element of size or mass. 

 To satisfy or balance the polarities of each order of physiological 

 units, not only a certain structure of organism, but a certain size 

 of organism is needed; for the complexities of that adult struc- 



