186 



AFFINITIES AND GEOGRAPHICAL 



We now proceed into the Vertebrata, of which the Fishes 

 compose the lowest class. Here, with a skeleton, are red 

 blood and a double-chambered heart ; but the blood is still 

 cold, and the respiration is carried on by branchise, the animals 

 being wholly designed for aquatic existence. 



Of the transitions or transmutations implied by the develop- 

 ment 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 may be supposed to have 

 been divided. Here, accordingly, we may expect to 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 wormlike in shape, 

 with only a rudirnental skeleton in the form of a horny or 

 gelatinous cord. They have a suctorial leechlike mouth, with 



FIG. 100. 



Lamprey : A, its circular suctorial mouth. 



numerous small teeth, by which they fasten upon living 

 animals for sustenance. The affinity to the cephalopods is 

 fully admitted. It is seen in the nature of the skeleton, in the 

 character of the investing skin which ejects a copious secretion 

 whenever the animal considers itself in danger, in the power 

 of respiring through the gill apertures without any dependence 



