THE FACTOKS OF OKGANIC EVOLUTION. 67 



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 expected 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.*&quot; 



Though it would enforce the argument, I must, for 

 brevity s sake, pass over the analogous evolution of that 

 introverted layer, or hypoblast, out of which the alimentary 

 canal and attached organs arise. It will suffice to emphasize 

 the fact that having been originally external, this layer 

 continues in its developed form to have a quasi-externality, 

 alike in its digesting part and in its respiratory part; since 

 it continues to deal with matters alien to the organism. 

 I must also refrain 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 antithesis which has great significance. 



* For a general delineation of the changes by which the development 

 is effected, see Balfour, I.e. Vol. ii, pp. 401-4. 

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