238 FROM FISH TO PHILOSOPHER 



remarkably thick, concentric walls; and an outer layer 

 of dense material laid down in laminae but penetrated 

 by radiating fibrous tissue, and capped superficially by 

 a layer of thick, enamel-like substance comprising the 

 tip of the tubercule. To these may have been added a 

 somewhat homy epithelial layer covering the enamel 

 caps and corresponding to the homy outer layer of the 

 later fishes and higher vertebrates {33, p. 102}. Bryant's 

 description does not add up to the destruction of the 

 value of the armor with respect to osmotic insulation, 

 but, on the contrary, to its inclusion as the impervious 

 covering of a soft-bodied animal that was capable of 

 growth and had to rebuild its armor as it grew. 



It may be emphasized that the eurypterids, co-in- 

 habitants of fresh water with the ostracoderms, were 

 also armored, though with a chitinous shell rather than 

 bony plates, and it can scarcely be argued that their 

 armor served to protect them against the bottom-living, 

 sluggish, jawless vertebrates. However, it may be sup- 

 posed that, as in the case of the vertebrates, their suc- 

 cessful invasion of fresh water was dependent on their 

 waterproofing. Since mutation must precede adaptation, 

 it is specious to argue whether the first steps toward the 

 development of armor occurred while the ancestors of 

 the vertebrates were still bona fide inhabitants of salt 

 water, or later when they were seeking squatters' rights 

 in the brackish lagoons. Escape from the osmotic argu- 

 ment cannot be sought with profit in the fact that even 

 the marine Cambrian invertebrates (including the trilo- 

 bites, ancestral to the eurypterids ) had sought protection 

 against predatory enemies or the vicissitudes of life 

 within calcareous or chitinous shells, because shifting 

 the origin of armor backward in time and phylogeny 

 merely emphasizes again that all evolution is preadap- 

 tiye: the favorable mutation always appears in at least 

 a potential form before, under the pressure of selection, 

 it proves to be advantageous for survival. The incon- 



