1076 LIFE : OUTLINES OF GENERAL BIOLOGY 



inches of toothed saw; so that the use, and even necessity, of the 

 sheath is obvious indeed. 



Another Mode of Classification. — On other Unes, illustrated 

 in many sections of this book, the numberless adaptations may be 

 arranged according to the advantages which they help to secure or 

 the difficulties which they meet, (a) Some adaptations secure the 

 smoother and better working of the everyday functions of the body, 

 such as the quality of warm-bloodedness. (b) Others are concerned 

 with the quest for food, such as the peculiarly silent flight of the 

 owl. (c) Others make for success in winning mates, like the odori- 

 ferous scales of some male butterflies, {d) Others help in successful 

 parentage, like the pouch of most mother-marsupials, (e) Others 

 again facilitate communal life, such as recognition-marks and kin- 

 signals. (/) Some have their chief reference to the avoidance of 

 enemies, such as cryptic colouration, death-feigning, autotomy. 

 (g) Some are specially related to the different seasons of the year. 



Fig. 190. 



Abyssal Fish (Gastrostomus), with large head, relatively enormous gape, and 

 locomotor body. Illustrating adaptation to the severe nutritive conditions 

 of the deep sea. After Murray. 



such as winter-blanching and summer aestivation, (h) Others again 

 have their significance in connection with particular habitats, such 

 as the tactility of Deep-sea animals. This is a kind of classification 

 which the reflective student readily elaborates. 



IMPERFECT ADAPTATIONS.— As the evolution of some struc- 

 tures and types has lasted for a relatively short time as compared 

 with its age-long duration in other cases, it is not surprising that 

 some adaptations are less perfect than others. The eyes of some 

 marine worms are like prentice-work, those of Alciopids are very 

 elaborate. Warm-bloodedness is imperfect in some mammals. The 

 specialisation of the dentition in carnivores shows many gradations. 

 It must be allowed that all adaptations are not equally perfect, and 

 that there is a tax to pay on certain advantages. 



It is doubtful, however, whether many instances can be found of 

 the persistence of the positively disadvantageous, except during the 

 period preceding the extinction of a type whose evolutionary 

 momentum has carried it beyond the line of safety. 



The enormous antlers of the Giant Irish Stag were probably 

 disadvantageous masculine exuberances, and it is likely enough that 

 the male narwhal finds its eight-foot long tusk somewhat embarrass- 



