EVOLUTION AND DEGENERATION OF ORGANS 27 



The organ does not, however, lose its primitive 

 and typical characters. Actual organic transforma- 

 tion cannot, therefore, be said to take place in the 

 case of individual adaptation. 



The same leaves of the same plant when grown in water are 

 much longer than those of the terrestrial type ; the leaves have no 

 stomata, and the epidermic cells are full of chlorophyll (Askenasy, 

 Ueber den Einfluss des Wachstumsmediiims auf die Gcstalt der 

 PJlanzen, Bot. Zeit., 1870, pp. 193 and following). Among the 

 Stratiotes aloides it is not uncommon for the upper part of a leaf to 

 rise above the surface of the water while the base is submerged ; the 

 epidermis at the base contains chlorophyll, but has no stomata, 

 while the part of the same leaf which rises out of the water is 

 furnished with stomata, but has no chlorophyll in the epidermis. 



(b) A good example of this individual adaptation may be obtained 

 by cultivating Cacti alternately in the light and in the dark. 

 Goebel has shown that when a specimen of Phyllocactus is culti- 

 vated in the dark, the stems are prismatic and thorny ; if the plant 

 is afterwards placed in the light, the thorns disappear and the 

 stems become quite smooth. (K. Goebel, Uebcr die Einwirkung des 

 Lichtes auf die Gestaltung der Kacteen und anderer Pflanzen, Flora, 

 vol. 80, p. 96, 1895.) 



(c) The animal kingdom furnishes numerous examples of indi- 

 vidual adaptation. 



The gradual drying up of Lake Aral caused the formation of a 

 number of basins containing water at various stages of concentration. 

 The Cardium of this region exhibits a whole series of adaptive varia- 

 tions. The shells become thinner and horny, their shape elongates, 

 the openings contract, and their colour becomes duller (Baieson). 



Mytilus edulis (the edible mussel), exhibits three different kinds 

 of shells. It lives either in salt water, deep water, or shallow 

 water visited by the tide. In each of these three vicinities the 

 shells exhibit typical aspects. 



The direction of boney lamellae is known to agree with that in 

 which the greatest strain is habitually applied, and the entire 

 structure of a bone is dominated by the incidences of the forces 

 applied to it. When, after a badly-mended fracture, the two broken 



