CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 53 



which unlocks whatever mysteries hedge about the fundamental 

 likeness we see in each type of animal life. 



In a future chapter we shall endeavour to trace the evidence 

 which has already been gathered in favour of the accumulation of 

 transitional forms, between the main divisions of the vertebrate type, 

 as illustrative of " missing links " at large. In the present instance 

 we may sum up the testimony which tends to support and prove 

 the biological declaration that, between the types themselves, there 

 exist intermediate forms, the presence of which tends to substitute 

 the idea of the gradual and continuous nature of animal development 

 as opposed to that of interrupted or " special creations." For 

 example, it is a comparatively easy matter to demonstrate that the 

 gulf between Vertebrate animals and their Invertebrate neighbours 

 has been largely bridged over, so that to-day no competent 

 naturalist doubts the connection of the highest type of animal life 

 with lower forms. The evidence of such a connection will be more 

 fully detailed hereafter, but it is permissible to refer to its main 

 details in the present instance. The Vertebrate animals have already 

 been shown to be those which alone possess a spine enclosing the 

 nervous system, and which, moreover, of all animals, are those 

 having the heart lowest, and possessing never more than four limbs, 

 these latter appendages being developed in pairs. But when we 

 pass to the lower confines 

 of this group, we discover 

 that the lowest fish (the 

 lancelet [or Amphioxus\, 

 Fig. 13) presents us with 

 a clear-bodied organism, 

 attaining a length of only 

 an inch or two, and desti- 

 tute of nearly all the 

 special belongings of the 

 fishes themselves. In 

 place of a spine and skeleton, it possesses a soft cellular rod (the 

 notochord), such as every other vertebrate develops in early life, 

 but which in all, save a few fishes, is replaced by the backbone. 

 It breathes by an enlargement of the throat ; its nervous system, 

 lying upon the "notochord," is a mere nervous cord destitute 

 of a brain ; its eyes are mere specks of colour ; and it wants a heart, 

 kidneys, spleen, and also the sympathetic nervous system found in all 

 other vertebrates. When, therefore, we attempt to place the lancelet 

 in an animal type, we are met by the difficulty that whilst, in the pos- 

 session of certain important characters, it is undoubtedly a vertebrate, 

 in the want of other characters it appears to lie outside that type. 

 Again, we discover an equally important fact when we learn that 



FIG. 13. LANCELET. 



a, head ; b, the fish viewed from the side ; c, filaments 

 surrounding the mouth. 



