2oo THE POPULAR SCIENCE MONTHLY. 



the special-creationist, would appear such a filiation of structures, and 

 such a round-about mode of embryonic development, we have here to 

 remark that the process is not one to have been anticipated as a result 

 of natural selection. After numbers of spontaneous variations had 

 occurred, as the hypothesis implies, in useless ways, the variation which 

 primarily initiated a nervous centre might reasonably have been ex- 

 pected to occur in some internal part where it would be fitly located : 

 its initiation in a dangerous place and subsequent migration to a safe 

 place, would be incomprehensible. Not so if we bear in mind the 

 cardinal truth above set forth, that the structures for holding converse 

 with the medium and its contents, arise in that completely superficial 

 part which is directly affected by the medium and its contents ; and 

 if we draw the inference that the external actions themselves initiate 

 the structures. These once commenced, and furthered by natural 

 selection where favourable to life, would form the first term of a series 

 ending in developed sense organs and a developed nervous system.* 



Though it would enforce the argument, I must, for brevity's sake, 

 pass over the analogous evolution of that introverted layer, or hypo- 

 blast, out of which the alimentary canal and attached organs arise. 

 It will suffice to emphasize the fact that having been originally exter- 

 nal, this layer continues in its developed form to have a quasi-exter- 

 nality, alike in its digesting part and in its respiratory part ; since it 

 continues to deal with matters alien to the organism. I must also re- 

 frain from dwelling at length on the fact already adverted to, that 

 the intermediate derived layer, or mesoblast, which was at the outset 

 completely internal, originates those structures which ever remain 

 completely internal, and have no communication with the environment 

 save through the structures developed from the other two : an antithe- 

 sis which has great significance. 



Here, instead of dwelling on these details, it will be better to draw 

 attention to the most general aspect of the facts. Whatever may be 

 the course of subsequent changes, the first change is the formation of 

 a superficial layer or blastoderm ; and by whatever series of transfor- 

 mations the adult structure is reached, it is from the blastoderm that 

 all the organs forming the adult originate. "Why this marvellous 

 fact ? "Why out of the primitive mass of organizable substance which 

 is to form a new creature, should its surface be the part from which is 

 remotely derived its entire structure ? Before embryologists had es- 

 tablished this truth, anyone who had asserted it would have been 

 thought insane ; and even now it remains a mystery if we refuse to 

 take account of the direct relations between the organism and the 

 medium. But we need only consider the incidents of this relation to 

 get a feasible explanation. Before yet the primitive metazoon had any 

 structure beyond that possessed by its component cells, its outer sur- 



* For a general delineation of the changes by which the development is effected, see 

 Balfour, I.e. Vol. ii, pp. 401-4. 



