ORGANIC STRUCTURE: MORPHOLOGICAL 51 



In this case (the fish-gill and mammalian lung) the essential 

 mechanism is a red-blood-corpuscle, containing haemoglobin, that 

 absorbs oxygen. In the fish-gill there is oxygen in the water that 

 bathes the membranes behind which is the fish blood-stream. 

 In the mammalian lung there is oxygen in the cavity that is 

 lined by a membrane, behind which is the mammal's blood- 

 stream. 



146. Unessential Structure. Details of animal structure 

 may be quite unessential to the bodily behaviour or to organic 

 functioning. Such details are, in a w^ay, superfluous, or excess, 

 structural details. For example, the Herring and Pilchard have 

 the same general habits and methods of nutrition and they have 

 analogous migrations and nearly similar modes of reproduction. 

 Yet the Pilchard has large and relatively thick scales while the 

 Herring has much smaller ones. Such a difference in structure 

 has no counterpart in functioning that we know, yet it is constant 

 and it is perfectly diagnostic of the specific, morphological 

 diiTerences in the two species of fish. 



i^c. Chemical and Morphological Structure. Only in 

 the most general way does chemical composition determine 

 morphological structure. In the large vertebrates (whales, the 

 extinct Dinosaurs, etc.) a very strong internal skeleton is necessary 

 to support the weight of flesh. Such a skeleton must involve 

 some such substance as lime, silica or perhaps ferrous hydroxides 

 or carbonates. Lime, of course, is actually used. 



But vast numbers of zoophytes have chitinous exoskeletons 

 and the chemistry of the soft parts of their bodies is the same in 

 all species, so far as we know. Yet the morphological differences 

 in these groups are very great. 



It is easy to see that there need not be an invariable relation 

 between chemical composition and animal morphological char- 

 acters. By analogy we may make this clear (for the chemistry 

 of the animal body is inconclusive in regard to the problem). 

 Very many patterns may be stamped out on the same coin blanks 

 and it is the construction of the dies, not the nature of the metallic 

 discs, that determines the patterns. 



Yet, on similar analogies, there may be a necessary relation 

 between structure and material : it would not, obviously, be 

 possible to construct such a fabric as the Eiffel Tower from 

 bricks and mortar. 



