^06 APPENDIX D. 



ture in which the physiological units are equilibrated, cannot b? 

 represented within the small bulk of the embryo. In many minute 

 organisms, where the whole mass of physiological units required 

 for the structure is present, the very thing does take place which 

 it is above implied ought to take place. The mass builds itself 

 directly into the complete form. This is so with Acari, and 

 among the nematoid Entozoa. But among higher animals such 

 direct transformations cannot happen. The mass of physio- 

 logical units required to produce the size as well as the structure 

 that approximately equilibrates them, is not all present, but has 

 to be formed by successive additions — additions which in vivipa- 

 rous animals are made by absorbing, and transforming into these 

 special molecules, the organizable materials directly supplied by 

 the parent, and which in oviparous animals are made by doing 

 the like with the organizable materials in the " food-yelk," de- 

 posited by the parent in the same envelope with the germ. Hence 

 it results that, under such conditions, the physiological units which 

 first aggregate intD the rudiment of the future organism, do not 

 form a structure like that of the adult organism, which, when 

 of such small dimensions, does not equilibrate them. They dis- 

 tribute themselves so as partly to satisfy the chief among their 

 complex polarities. The vaguoly -differentiated mass thus pro- 

 duced cannot, however, be in equilibrium. Each increment of 

 physiological units formed and integrated by it, changes the dis- 

 tribution of forces ; and this has a double effect. It tends to 

 modify the differentiations already made, bringing them a step 

 nearer to the equilibrating structure ; and the physiological units 

 next integrated, being brought under the aggregate of polar forces 

 exercised by the whole mass, which now approaches a step nearer 

 to that ultimate distribution of polar forces which exists in the 

 adult organism, are coerced more directly into the typical struc- 

 ture. Thus there is necessitated a series of compromises. Each 

 successive form assumed is unstable and transitional : approach 

 to the typical structure going on hand in hand with approach to 

 the typical bulk. 



Possibly I have not succeeded by this explanation, any more 

 than by the original explanation, in making this process " repre- 

 sentable in thought." It is manifestly untrue, however, that I 

 have, as alleged, re-introduced under a disguise the conception of 

 a " vital principle." That I interpret embryonic development in 

 terms of Matter and Motion, cannot, I think, be questioned. 

 Whether the interpretation is adequate, must be a matter of 

 opinion; but it is clearly a maHer of fact, that I have not fallen 

 into the inconsistency asserted by your reviewer. At the same 

 time I willingly admit that, in the absence of certain statements 

 which I have now supplied, he was not unwarranted in represent- 

 ing my conception in the way that he has done. 



(1) 



