STKATinCATION IN STEUCTURE. 31 



tially on the axiom that the topographical relations of the in- 

 dividual organs must always be of the same general type, not- 

 withstanding the utmost variety in the forms of the organs 

 themselves or of the animals to which they belong, so long as 

 these are included in the same systematic group. Thus, for 

 instance, a vertebrate animal with a brain in the foot, as some- 

 times occurs in MoUusca, or with an ear in its tail, as in cer- 

 tain Crustaceans, is simply an impassibility ; among the Ver- 

 tebrata these organs are always necessarily located in the skull, 

 or brain-capsule, which is invaiiably in the head ; every organ, 

 even the most insignificant, as it might appear, has its deter- 

 mined position from which it is but rarely displaced. This 

 topography or distribution of the organs is indicated and recog- 

 nisable at a very early period in the life of the individual. Be- 

 fore any kind of organ is constructed and adapted to any 

 determined use, two or three embryonic layers or strata of 

 cells are formed (fig. 11), known to embryologists as the 

 germinal layers. Each of these is gradually formed or dififer- 

 entiated into certain organs. Thus, for instance, it is now 

 ascertained that the central nerve-system originates in almost 

 all animals directly from the outer layer, the so-called ' Ectoderm,' 

 of the embryo ; and we know also that the eyes and ears are 

 formed in the same way from this outer germinal layer, and at 

 the same time from the central nervous system, which has been 

 already separated from it. The same obtains of all the other 

 organs, of which some always originate from the inner germinal 

 layer, others again as invariably from the median layer. This 

 stratification of the body, which is, on the whole, tolerably 

 uniform in all animals, and the early appearance of the three 

 principal germinal layers, can no longer be doubted 



If we now look at a map of the world on which the dis- 

 tribution of the fauna into districts is indicated by different 

 colours, and compare this with the lists of Birds, Mammal a, 

 or Reptiles which usually accompany such a map, we perceive 

 that a great number of species, genera, and even families occur 

 in only one district and not at all in the others. This matter 

 has lately been admirably treated by the distinguished English 

 naturalist Wallace, whose work on the geographical distribution 



