196 PRESENT-DAY RATIONALISM 



So, too, we may apply this line of argument to existing 

 plants and animals. The difficulty only lies in selecting from 

 the enormous mass of such evidence at our disposal. 



Since it has been found experimentally how easily most 

 plants will change their structures and forms as soon as they 

 are grown in very different soils and climates from those of 

 their natural habitats, one naturally infers that they will behave 

 in Nature in a way analogous to that which they do under 

 cultivation ; provided the change of environment can furnish 

 a sufficient stimulus, as does a richly prepared garden soil. 



Linnaeus long ago (1763), noticed that when he studied 

 batches of plants received from foreign countries, there was 

 often a certain superficial fades or likeness between many of 

 them ; though there need not have been any real affinities : 

 " Primo intuitu distinguit saepius exercitatus botannicus plantas 

 Africae, Asiae, Americae, Alpiumque, etc. " ^ 



Modern botanists have applied the term "representative" 

 to plants when they live in widely different countries and may 

 be either different species of the same genus or plants of no 

 affinity, but assume similar vegetative forms or physiognomy. 



Thus many members of the family Cactacece. inhabiting hot, 

 dry, rocky districts of Mexico, strikingly resemble species of 

 the genus Euphorbia in the hot but dry regions of Africa, as 

 well as the Stapelias of South Africa and Adenium of Arabia, etc. 

 All these, though belonging to totally different families, yet 

 have similar columnar, thick, fleshy, leafless but spiny stems. 



In all species of Euphorbia living in Europe, there is no 

 tendency whatever to produce the cactus-like stem. It only 

 appears under similar climatic and other conditions of life in 

 which the cacti themselves live. 



The induction, therefore, is that, first, the tendency to pro- 

 duce the fleshy stem has arisen through the influences or 

 "direct and definite action," as Darwin called it, of the 

 specially dry environment. 



But this tendency has now become z fixed character, im- 



' See Origin of Plant Structures, p. 13. 



