DISTRIBUTION OF OBGANISMS. 163 



In looking among the animals of one class for the point 

 of connexion by which it is joined to the next above, we 

 must not invariably expect to find what we are wanting in 

 the highest species, for these are often the heads of branches. 

 On the contrary, it appears, in many instances, in the lower 

 species. And this is the more worthy of being pointed out, 

 as the supposition of something different has supplied many 

 of the stumbling blocks of the development theory. In all 

 the classes, for instance, which have terrestrial as well as 

 marine species, the nexus to the next grade of being is among 

 the latter, which are invariably the inferior. And thus it is 

 that no transition of the kind here under our attention that 

 is, none of the greater grade transitions takes place out of 

 the aquatic medium which I have regarded as analogous to 

 that of all individual embryos. 



We now proceed into the Vertebrata, of which the fishes 

 compose the lowest class. Here, with a skeleton, we have 

 red blood and a double chambered heart ; but the blood is 

 still cold, and the respiratory system is by branchiae, the 

 animals being wholly designed for aquatic existence. 



Of the transitions or transmutations implied by the deve- 

 lopment theory, the greatest, or most violent, are those few 

 which took place in the passage from the invertebrate animals 

 to fishes, from fishes to reptiles, and from these to the higher 

 classes. This we might expect, as at such points the pheno- 

 menon had nothing to do with external circumstances, but 

 wholly depended on the internal development force, each 

 stage being one of that limited number of periods, into which 

 the long enduring gestation of nature was divided. Here, 

 accordingly, we shall always find the affinities less distinct 

 than elsewhere ; and yet at all of them some connexions are 

 visible, leaving the general fact of the transition indubitable. 



Between the invertebrate animals and the fishes, the junc- 

 tion is tolerably clear at one point. This is where the cepha- 

 lopodous mollusks connect with such fishes as the myxine, 

 or hag, and the lamprey. These fishes are worm-like in 

 shape, with only a rudimental skeleton in the form of a horny 

 or gelatinous cord. They have a suctorial leech-like mouth, 



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