INTRODUCTORY ESSAY. xi 
or by that of any other plant,* they eventually revert to one of their parents : on the other hand, 
the number of varieties is incalculable, the power to vary further is unimpaired in their progeny, 
and these tend to depart further and further in sensible properties from the original parent. 
In conformity with my plan of starting from the variable and not the fixed aspect of Nature, 
I have now set down the prominent features of the Vegetable Kingdom, as surveyed from this point 
of view. From the preceding paragraphs the evidence appears to be certainly in favour of prononess 
to change in individuals, and of the power to change ceasing only with the life of the individual ; 
and we have still to account for the fact that there are limits to these mutations, and laws that con- 
trol the changes both as to degree and kind; that species are neither visionary nor even arbitrary 
creations of the naturalist ; that they are, in short, realities, whether only temporarily so or not. 
13. Granting then that the tendency of Nature is first to multiply forms of existing plants by 
graduated changes, and next by destroying some to isolate the rest in area and in character, Ave are 
now in a condition to seek some theory of the modus operandi of Nature that will give temporary 
permanence of character 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 demon- 
strating the effects of these in the production of any one species in a native state. 
Of these speculations by far the most important and philosophical is that of the delimitation 
of species by natural selection, for which we are indebted to two wholly independent and original 
thinkers, Mr. Darwin and Mr. Wallace.t These authors assume that all animal and vegetable forms 
are variable, that the average amount of space and annual supply of food for <aeli species (or other 
group of individuals) is limited and constant, but that the increase of all organisms tends to proceed 
annually in a geometrical ratio ; and that, as the sum of organic life on the surface of the globe does 
not increase, the individuals annually destroyed must be incalculably great ; also that each species is 
ever warring against many enemies, and only holding its own by a slender tenure. In the ordinary 
course of nature this annual destruction falls upon the eggs or seeds and young of the organisms, 
and as it is effected by a multitude of antagonistic, ever- changing natural causes, each more destruc- 
tive of one organism than of any other, it operates with different effect on each group of individuals, 
in every locality, and at every returning season. Here then we have an infinite number of varying 
conditions, and a superabundant supply of variable organisms, to accommodate themselves to these 
conditions. Now the organisms can have no power of surviving any change in these conditions, 
except they are endowed with the means of accommodating themselves to it. The exercise of 
this power may be accompanied by a visible (morphological) change in the form or structure of the 
individual, or it may not, in which case there is still a change, but a physiological one, not outwardly 
* A very able and carefid experimenter, M. Naudin, performed a series of experiments at the Jardin des Plantes 
at Paris, in order to discover the duration of the progeny of fertile hybrids. He concludes that the fertile posterity 
of hybrids disappears, to give place to the pure typical form of one or other parent. " II se peut sans doute qu'il 
y ait des exceptions a cette loi de retour, et que certains hybrides, a la fois tres-fertiles et tres-etablis, tendent a faire 
souche d'espece ; mais le fait est loin d'etre prouve'. Plus nous observons les phe'nomenes d'hyhridite, plus nous 
inclinons a croire que les especes sont indissolublement liees a une fonction dans l'ensemble des choses, et que e'est 
le role nienie assigne a chacune d'elles qui en determine la forme, la dimension et la duree." (Annales des Sc Nat 
ser. 4. v. 9.) 
f Journal of the Linnean Society of London, Zoology, vol. iii. p. 45. 
c 2 
