THE GEOGKArHICATv DISTRIBUTION OF PLANTS. 117 



Tve have just considered, — which groups probably all came in a 

 series of great waves from the north. 



Discussion. 



William C. Strong said the lecturer had spoken of the transpor- 

 tation of plants by glacial action and of the gradual distribution 

 of species, and asked him whether he regarded the law of evolution 

 as applicable to plants in such manner as to increase their hardi- 

 ness, enabling seedlings of species belonging in the flora of a 

 warm climate, ultimately, by a long process, to become inured to a 

 colder climate. 



Mr. Ganong replied that the gradual modification of the 

 character and habits of plants was so general and so necessary ta 

 fit them to their surroundings in localities where they may be 

 placed either b}" design or seeming accident, that the known facts 

 could not be explained without supposing it, and it may be taken, 

 to start with, as almost axiomatic. 



F. L. Temple inquired whether the plants that were driven 

 south, by the glaciers, far into the tropical zone and had returned, 

 by Nature's slow process, to their original home, were then iden- 

 tical with their ancestors growing thei'e in pre-glacial times. 



Mr. Ganong replied that practical!}' they were identical ; that, 

 although somewhat modified by the various influences to which 

 they had been exposed during their absence, the intervening time 

 had not been long enough to extinguish the characteristics of the 

 species, some of which indeed are very old and are found fossil in 

 the Tertiary strata. 



Mr. Temple asked the reason that plants brought from the 

 southern extremity of South America will not live here in our 

 climate, or in the degree of latitude north, corresponding to their 

 native habitat. 



Mr. Ganong suggested that if plants were rightly selected in. 

 Fuegia they might live here. But the chances are against it in. 

 such cases, unless a gradual acclimatization can be effected. Special 

 conditions, other than those mentioned of moisture and tempera- 

 ture, even if so slight as to be invisible or unknown to us, may 

 become, in the case of wild plants, of vital importance in the face 

 of the intense competition between species each striving to occupy 

 as much ground as possible. The native plants are in possession 

 and the foreigners are not sufficiently adapted to our climate to 



