196 ON THE INTERNAL FORM AND [ch. 



been observed, and are not even specially and indisputably con- 

 nected with the organism as such. They include such manifesta- 

 tions of the physical forces, in their various permutations and 

 combinations, as may also be witnessed, under appropriate 

 conditions, in non-living things. 



When we attempt to separate our purely morphological or 

 "purely embryological" studies from physiological and physical 

 investigations, we tend ipso facto to regard each particular structure 

 and configuration as an attribute, or a particular "character," of 

 this or that particular organism. From this assumption we are 

 apt to go on to the drawing of new conclusions or the framing of 

 new theories as to the ancestral history, the classificatory position, 

 the natural affinities of the several organisms : in fact, to apply 

 our embryological knowledge mainly, and at times exclusively, to 

 the study of phytogeny. When we find, as we are not long of 

 finding, that our phylogenetic hypotheses, as drawn from em- 

 bryology, become complex and unwieldy, we are nevertheless 

 reluctant to admit that the whole method, with its fundamental 

 postulates, is at fault. And yet nothing short of this would 

 seem to be the case, in regard to the earlier phases at least of 

 embryonic development. All the evidence at hand goes, as it 

 seems to me, to shew that embryological data, prior to and even 

 long after the epoch of segmentation, are essentially a subject for 

 physiological and physical investigation and have but the very 

 slightest link with the problems of systematic or zoological 

 classification. Comparative embryology has its own facts to 

 classify, and its own methods and principles of classification. 

 Thus we may classify eggs according to the presence or absence, 

 the paucity or abundance, of their associated food-yolk, the 

 chromosomes according to their form and their number, the 

 segmentation according to its various "types," radial, bilateral, 

 spiral, and so forth. But we have little right to expect, and in 

 point of fact we shall very seldom and (as it were) only accidentally 

 find, that these embryological categories coincide with the lines 

 of "natural" or "phylogenetic" classification which have been 

 arrived at by the systematic zoologist. 



The cell, which Goodsir spoke of as a "centre of force," is in 



