NATURAL SELECTION V. ADAPTATION 



81 



th?in, also in very dilute solutions made up to the same osmotic pressure 

 with KC1 and also Ca(N0 3 ) 2 . These were allowed to evaporate down, 

 becoming gradually stronger. When they reached a strength of solution 

 equal to about ^ N salt solution the water leaves ceased to form and the 

 air type of leaf appeared." * 



I shall now give a good instance of inductive evidence in support of 

 self-adaptation in marine animals. One of the commonest means cf 

 propulsion is the "paddle " as employed in canoes and in the form of the 

 oar in boats. Looking through the animal kingdom from the earliest 

 times till now we find it has been very generally adopted by nature in 

 several classes, as in Crustacea, fishes, extinct marine reptiles, modem 

 cetacea, in the tail of the beaver, and even in birds, as the aborted wings of 

 penguins and auks. As these classes have nothing of the nature of 

 affinity between them, the conclusion drawn from so many coincidences is 

 that the paddle," though made in various ways, is the definite result of the 

 effort of the animal made in swimming, together with the mechanical cr 

 direct action of the water. Similarly the bilobed tail of a fish is imitated 

 in extinct fossil marine reptiles and in modern whales and porpoises. 



Numerous and similar series of coincidences can be drawn up from the 

 animal kingdom, proving inductively that there has been a direct cause 

 and effect between the animal and its habits with its environment. 



Such are exactly parallel with the inference drawn from submerged 

 leaves as described : It is not possible to corroborate inductive evidence 

 in the animals mentioned by means of experiment, as in the case of 

 Proserpinaca palustris, but the evidence is equally conclusive. 



Nature herself, however, often performs similar experiments, as when 

 crowded a shoot of the Water Crowfoot is forced to grow up into the air, cr 

 when a pond dries up and it has to grow in the mud only. Then, though 

 the form of the dissected leaf is still retained by heredity, the segments 

 are now rigid ; they also abound with stomata, and all the anatomical 

 details are now suited for an aerial existence. 



As illustrations of plants acquiring the same forms under the opposite 

 conditions of great drought, I need only mention the Cactacece of Mexico 

 imitating Euphorbias and Stapelias of South Africa in their massive fluted 

 and spiny stems devoid of leaves. Similarly Agave of America resembles 

 Aloe of Africa. 



Although Darwin founded his theory of natural selection on obser- 

 vations made on animals and plants under domestication, and was 

 thereby misled as to "indefinite variation," nevertheless cultivation gives 

 us the best illustrations of the way plants change in response to the 

 definite action of the environment. Thus it is a familiar fact with cul- 

 tivators that when native or wild plants are first introduced into garden? 

 they often change so much that they even may assume all the appearance 

 of a distinct species. For example, Arab is anchoret ica, a plant with thin 

 papery leaves, frequenting rocks in Switzerland, when grown at Kew 

 resembles A. alpina, and it is the experience of a celebrated bulb-grower that 

 many which came from the East could scarcely be recognised after a few 



* " On the Nature of the Stimulus causing the Change of Form and Structure "k 

 Proserpinaca palustris,'" Bot. Gaz, vol. xxxiv. p. 93. 



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