ANIMALS AND THEIR ENVIRONMENTS. 



237 



been found to acquire unsymmetrical skulls from 

 this habit ; and they have also been seen to strain 

 their lower eyes in the endeavor to look upward, 

 after the fashion of the young flatfish. One au- 

 thority, indeed, declares that it is possible that 

 the young of the most modified flatfishes are in 

 reality unsymmetrically developed even within the 

 egg. This condition, if actually present, must 

 necessarily be viewed as the inherited result of 

 the typical development of the unsymmetrical 

 state in ancestral forms ; and its occurrence would 

 render easy of explanation the cause of the young 

 fish losing its balance so soon after its escape 

 from the egg. In some fishes, which are widely 

 removed ■ in their systematic position from the 

 flatfishes, there is a want of symmetry which 

 compels the fishes to rest on one side. Such are 

 the curious deal-fishes {Trachypterus arcturus) or 

 vaagmiirs, which derive their popular name from 

 their exceeding thinness of body, and which are 

 allied to the familiar tape or ribbon fishes. The 

 deal-fish rests on its left side, and, like the flat- 

 fishes, is a bottom-living species. Moreover, it 

 swims diagonally through the water from its 

 want of symmetry, and evinces a disparity in 

 development between the two sides of the head. 

 The occurrence of allied conditions in the heads 

 of higher animals and in other and distinct groups 

 of fishes, would seem to argue clearly and forci- 

 bly in favor of like conditions producing like 

 results to those seen in the flatfishes. Nor must 

 we lose sight of the fact that disuse of the fins 

 of the lower side in the flatfishes will account for 

 their lesser size, as compared with those of the 

 upper surface ; and that the jawbones are strong- 

 er and teeth more numerous on the lower side 

 of the head. This latter result accrues naturally 

 from the more constant use of the jaws on the 

 lower side of the head than on the eyed side in 

 the act of feeding on the ground — a fact pointed 

 out by my friend Dr. Traquair, and illustrating 

 the influence of " use " in developing structures, 

 as opposed to the effects of " disuse " in render- 

 ing organs useless and abortive. From every 

 consideration we are forced to conclude that the 

 flatfishes present us with typical examples of ani- 

 mals which owe their peculiar form and habits to 

 the circumstances of their life, associated with 

 the action of environments upon their frame. 

 We learn from the consideration of such features 

 of living beings, not only how perfectly adapta- 

 tion to circumstances is correlated with structure 

 and life at large, but also how plastic and mobile 

 under the sway of outward forces the living or- 

 ganism may prove. While no less powerfully 



does the consideration of the flatfishes and their 

 modifications support the ideas that the existing 

 order of Nature is largely due to secondary causes 

 and to mechanical forces which acquire domi- 

 nance and power over living beings through the 

 effects of perpetuated habit, and of use or disuse 

 continued through long periods of time. 



Within the confines of the group of vertebrate 

 animals ranking next in order to that of the fishes, 

 we may find examples of the relationship between 

 living beings and their surroundings, if anything, 

 of more typical and distinct nature than those 

 presented by the flatfishes. This group of ani- 

 mals is known as that of the Amphibia, and is 

 represented by the frogs, toads, newts, and allied 

 animals, which, in popular phraseology, would be 

 termed " reptiles," although zoologically they form 

 a perfectly distinct group from the latter creatures. 

 It may facilitate the comprehension of the illus- 

 trations about to be brought forward, if we firstly 

 glance at certain of the chief characters by which 

 the class of amphibians is distinguished. The 

 newts, frogs, toads, and their allies, without ex- 

 ception, pass through a series of changes in form 

 (or metamorphosis) in their young condition, and 

 possess breathing-organs in the form of external 

 gills in early life ; facts well known to any one 

 who has seen a young frog in its tadpole stage, 

 and who has had the curiosity to watch the trans- 

 formation of the tadpole into the adult frog. All 

 amphibians further possess lungs in their fully- 

 grown condition, whether the gills of early life 

 persist or not. Thus, the curious lizard-like Pro- 

 teus, found in the caves of Adelsberg, and the still 

 more curious Axolotl of Mexico, exemplify newt- 

 like creatures which retain the gills of early life, 

 and breathe by these organs as well as by the 

 lungs with which they are provided in their adult 

 shape. The common newts of our ponds and 

 ditches, the land-newts of other countries, and 

 the frogs and toads, breathe, on the contrary, by 

 lungs alone in their perfect condition ; the gills 

 of early life being discarded when these creatures 

 assume terrestrial habits. Thus, the newts, al- 

 though living essentially in water, breathe like the 

 frogs by lungs alone in their adult state ; and, like 

 the aquatic and lung-bearing whales, have to as- 

 cend periodically to the surface for a supply of at- 

 mospheric air. Bearing these characters of this 

 group of animals in mind, the curious nature of 

 the changes through which certain of its mem- 

 bers pass may be fully realized. The axolotl 

 (Siredon pi sci forme) is a creature inhabiting the 

 fresh waters of Mexico, and, despite its some- 

 what uninviting appearance, is used in its native 



