METHODS FOR PROVING ORIGIN OF SPECIES. 13 



Geographical botany teaches us what countries have 

 genera and even species in common, in spite of a certain 

 distance, and what, on the contrar}^, are very different, 

 in spite of similarity of climate or inconsiderable dis- 

 tance. It also teaches us what species, genera, and 

 families are scattered over a wide area, and the more 

 limited extent of others. These data are of gi'eat assist- 

 ance in determining the probable origin of a given 

 species. Naturalized plants spread rapidly. I have 

 (juoted examples elsewhere^ of instances within the last 

 two centuries, and similar facts have been noted from 

 year to year. The rapidity of the recent invasion of 

 Anacharis Alsinastruvi into the rivers of Europe is well 

 known, and that of many European plants in New 

 Zealand, Australia, California, etc., mentioned in several 

 Horas or modern travels. 



The great abundance of a species is no proof of its 

 anti(][uity. Agave Americana, so common on the shores 

 of the Mediterranean, although introduced from America, 

 and our cardoon, which now covers a great part of the 

 Pampas of La Plata, are remarkable instances in point. 

 As a rule, an invaaing species makes rapid way, while 

 extinction is, on the contrary, the result of the strife of 

 several centuries asjainst unfavourable circumstances.^ 



The designation which should be adopted for allied 

 species, or, to speak scientifically, allied forms, is a 

 problem often presented in natural history, and more 

 often in the category of cultivated species than in others. 

 These plants are changed by cultivation. Man adopts 

 new and convenient forms, and propagates them by 

 artificial means, such as buddinfj, orraftinor the choice of 

 seeds, etc. It is clear that, in order to discover the origin 

 of one of these species, we must eliminate as far as possible 

 the forms which appear to be artificial, and concentrate our 

 attention on the others. A simple reflection may guide 

 this choice, namely, that a cultivated species varies 

 chiefly in those parts for which it is cultivated. The 

 others remain unmodified, or present trifling alterations, 



' A. de Candolle, Geographie Botanique Raisonnee, chap. vii. and x. 



Ibid., chap. viii. p. 8'I4. 



