RELATIONS OF ANIMAL MORPHOLOGY 251 



structure fits them, and not elsewhere, points to the close relations 

 existing between morphology and geography. We find the animals 

 of the seashore, such as sponges, hydro- and anthozoa, and tunicates, 

 to be largely sessile, and, in consequence, of the radiate type of 

 structure. This sessile habit makes it possible for them to maintain 

 their hold on the rocks from which the beating waves tend to tear 

 them. Those which are not actually permanently attached have 

 means enabling them to cling closely to the rocks; such are the ech- 

 inoderms, the mollusks, many annelids and crustaceans. The ani- 

 mals of the surface of the sea, such as siphonophores, ctenophores, 

 jelly-fishes and larvae, are without such clinging organs; they include 

 species whose organization permits them only to float or swim at or 

 near the surface. The deep sea could have been populated more 

 readily, so far as proximity goes, from the surface organisms than 

 from those of the shore-line; but only the latter offered the struc- 

 tural features consonant with life at the sea-bottom, and so the deep 

 sea became populated thence. In the swift-flowing rivers we find 

 powerful swimmers or animals that can hold fast to the bed of the 

 stream and in ponds, we find those species which have some means of 

 preserving their continuity in time of drought. In caves * we get not 

 any forms which happen to be washed into them, but only darkness-, 

 moisture-, and contact-lovers. In deserts whole groups of animals are 

 absent, only those occurring with thickest skin, least apt to lose 

 water by transpiration, such as certain snakes, lizards, and hard- 

 shelled beetles. 2 In general, in studying the geographical distribution 

 of animals in environments presenting extreme conditions we find 

 that they clearly have been selected from groups presenting the most 

 favorable characteristics. All of this indicates that, often at least, the 

 already existing morphological conditions have determined the fitness 

 of a species to cope with the environment morphological charac- 

 teristics have determined geographical and climatic distributions. 



Morphology as the science of form is often contrasted with physi- 

 ology, the science of function. Yet between the two is the closest 

 possible relation, because an organ implies a function, and every 

 morphological characteristic has a corresponding physiological char- 

 acteristic. As physiological characteristics are transmissible in the 

 same way as morphological, we may think of any individual as being 

 made up of such functional characteristics, just as a molecule is made 



1 Compare C. H. Eigenmann, Cave Animals : Their Character, Origin, and their 

 Evidence for or against the Transmission of Acquired Characters, Proceedings of 

 American Association for the Advancement of Science, Forty-eighth Meeting 

 (1899), p. 255. Also P. C. K. Absolan, Einige Bemerkungen iiber mahrische Hohlen- 

 fauna, Zoologische Anzeiger, xxm, 1-6 (1900). For a popular account of our 

 caves and their fauna, see W. S. Blatchley, Gleanings horn Nature, pp. 99-178 

 (1899). 



2 For a delightful and accurate general account of the animals of our native 

 deserts, see John C. Van Dyke, The Desert, New York, 1901. 



