14 J.D. Hooker, Introductory Essay to the Flora of Tasmania. : 
we are now in a condition to seek some theory of the modus — 
operand: of Nature that will give temporary permanence of char: — 
acter to these changelings. And here we must appeal to theory 
or speculation; for our knowledge of the history of species in — 
relation to one another, and to the incessant mutations of their — 
environing physical conditions, is far too limited and incomplete 
to afford data for demonstrating the effects of these in the pro- 
duction of any one species in a native state. 
Of these speculations by far the most important and philo- — 
sophical is that of the delimitation of species by natural selec- 
tion, for which we are indebted to two wholly independent and — 
original thinkers, Mr. Darwin and Mr. Wallace.* These authors 
assume that all animal and vegetable forms are variable, that the 
average amount of space and annual supply of food for each 
species (or other group of individuals) is limited and constant, 
but that the increase of all organisms tends to proceed ann 
ually — 
in a. geometrical ratio; and that, as the sum of organic life on ~ 
¥ 
the surface of the globe does not increase, the individuals an- 
nually destroyed must be incalculably great; also that each spe- 
cles is ever warring against many enemies, and only holding its 
own by aslender tenure. In the ordinary course of nature this — 
progeny of fertile hybrids. He concludes that the fertile a of hybrids dis- 
appears, to give place.to the pure typical form of one or other parent. “ 
loin d’étre prouvé. Plus nous observons les 4 eto @hybridité, plus nous in- 
clinous a croire que les espéces sont indissolu raent liées ti 
ass 
