THE FACTORS OF ORGANIC EVOLUTION. i 97 



a period of irritability, great at first but decreasing as the change ad- 

 vances, this membrane assumes the general character of ordinary skin. 

 Nor is this all : its microscopic structure changes. Where it is a mu- 

 cous membrane of the kind covered by cylinder-epithelium, the cylin- 

 ders gradually shorten, becoming finally flat, and there results a squa- 

 mous epithelium : there is a near approach in minute composition to 

 epidermis. Here a tendency towards completion of the type cannot 

 be alleged ; for there is, contrariwise, divergence from the type. The 

 effect of the medium is so great that, in a short time, it overcomes the 

 inherited proclivity and produces a structure of opposite kind to the 

 normal one. 



Fully to perceive the way in which these evidences compel us to 

 recognize the influence of the medium as a primordial factor, we need 

 but conceive them as interpreted without it. Suppose, for instance, 

 we say that the structure of the epidermis is wholly determined by 

 the natural selection of favourable variations ; what must be the posi- 

 tion taken in presence of the fact above named, that the cell-structure 

 of mucous membrane changes into the cell-structure of skin when 

 mucous membrane is exposed to the air ? The position taken must be 

 this : Though mucous membrane in a highly-evolved individual or- 

 ganism, thus shows the powerful effect of the medium on its surface ; 

 yet we must not suppose that the medium had the effect of producing 

 such a cell-structure on the surfaces of primitive forms, undifferen- 

 tiated though they were ; or, if we suppose that such an effect was 

 produced on them, we must not suppose that it was inheritable. Con- 

 trariwise, we must suppose that such effects of the medium either were 

 not wrought at all, or that they were evanescent : though repeated 

 through millions upon millions of generations they left no traces. 

 And we must conclude that this skin -structure arose only in conse- 

 quence of spontaneous variations not physically initiated (though like 

 those physically initiated) which natural selection laid hold of and in- 

 creased. Does any one think this a tenable position ? 



And now we approach the last and chief series of morphological 

 phenomena which must be ascribed to the direct action of environing 

 matters and forces. These are presented to us when we study the 

 early stages in the development of the embryos of the Metazoa in 

 general. 



We will set out with the fact already noted in passing, that after 

 repeated spontaneous fissions have changed the original fertilized germ- 

 cell into that cluster of cells which forms a gemmule or a primitive 

 ovum, the first contrast which arises is between the peripheral parts 

 and the central parts. Where, as with lower creatures which do not 

 lay up large stores of nutriment with the germs of their offspring, the 

 inner mass is inconsiderable, the outer layer of cells, which are pres- 

 ently made quite small by repeated subdivisions, forms a membrane 



