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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 

 archoeus, 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 ex- 

 plain 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 understood 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 condition 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 undulations of a certain strength and period ; 

 and, within limits, the rapidity with which the physiological units 

 pass from their indefinite arrangement to the definite arrangement 

 they presently assume, is proportionate to the strengths of the 

 etherial undulations falling upon them. In its complete form, then, 

 the conception is that these specific molecules, having the immense 

 complexity above described, and having correspondently 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 en- 

 vironing 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 deter- 

 mining the direction in which that power is turned. Into this con- 

 ception there enters no trace of the hypothesis of an " archanis 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 ' inherent 

 tendency.' Could we more truly say of anything, * it is unrepresent- 

 able in thought 1 ' I reply that if there is taken into account an 

 element here overlooked, the process will not be found " unrepre- 

 sentable 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 structure 



