24 NATURAL HISTORY OF VERTEBRATE ANIMALS 



by reproduction. There seems to be a less active period in the 

 late summer, followed by further activity preceding the hiberna- 

 tion. With the lowering of the temperature in the fall, the animal 

 goes to the bottom and works its way into the mud or under the 

 bank, where it remains dormant until spring. Shortly after its 

 emergence, the mating occurs. The eggs are laid by the female 

 during sexual union, and fertiHzed by the sperm of the male as they 

 pass into the water. At first each egg is surrounded only by a 

 thin layer of sticky substance. In a few hours, however, this 

 imbibes water and becomes the capsule of jelly surrounding the 

 individual eggs, which he massed together and hghtly attached to 

 submerged objects near the surface (Fig. 11 A). Development 

 proceeds, in the course of three or four months, through the 

 familiar tadpole stages to the miniature adult. The rather sudden 

 change from the tadpole to the young frog is termed the metamor- 

 phosis (Fig. 11 I-K). In the tadpoles, gills hke those of fishes con- 

 stitute the primary organs of respiration, although well-developed 

 lungs are present and assume an increasing importance in later 

 tadpole stages. With metamorphosis, the gill apparatus in part 

 disappears and is in part converted into other organs. The frog 

 tadpole thus resembles a fish, since it develops in the open water 

 and possesses certain fish-like organs. 



Adaptation. — As has already been indicated, certain activities 

 of the frog are so well fitted to the needs of the animal in its 

 struggle for life as to attract our attention. A like condition pre- 

 vails in other organisms. The leaves of a plant are adapted to 

 perform certain functions; the stems and roots, others. Animals 

 are adapted for many differing modes of hfe, each to its own set 

 of conditions. While it is sometmies argued that non-living things 

 also exhibit what may be termed adaptation, as when we find 

 hydrogen " adapted " to combine with oxygen in the formation 

 of water, the earth adapted to revolution about the sun, or the 

 stones in the bed of a stream adapted to their particular places, 

 this adaptation, or fitness, of inanimate objects is far less compli- 

 cated than that observed in living bodies. 



For convenience, the adaptations of organisms may be grouped 

 as: (1) Anatomical, or structural; (2) Physiological or func- 

 tional; and (3) Related to behavior. This, however, represents 

 no hard and fast distinction. It would perhaps be better to 

 say that structure and function are everywhere inter-related, 



