INTRODTJCTrON. 
vii 
production of something out of nothing. If, how(!V(;r, the iruiuiry be lirnitrd ir, tlie oi i'_riti 
of living organisms only, it docs not necessarily involve; iuiy thing f'nilhci- limn :i r h;in'_'c 
condition of matter; for the presence of inorganic substance is c(;rtaiiily far antcii'n- to thf; 
first dawn of life. A time must have been wlien tliis cliange took plac(! — wlien thf; l)r'';ith 
of life first appeared in the organisms on the; surfaces of tliis earth; and iis it h;is nnflonht- 
edly occurred once, it is within the limits of possibility that it may have occurred yg;iiii ;iiifl 
again ; and it may even agcount for the successive and, otherwise, too sudden appcarancf; of 
new orders and families. 
The majority of the experiments on which the theory of modified descent is based liave 
reference to animals &c. under domestication ; and in enumerating these sufticicnt weiglit dfjes 
not seem to have been attached to the fact that these domestic species do not exhilnt th(; 
hereditary permanence that distinguishes true species, and that, when the disturbing influence 
is removed and the former conditions of life are restored, they lose their identity and in process 
of time revert to the original type : as an instance of this the case of the dog is most notable, 
and must have attracted the attention of many people in India and the East. In countries 
where the wild dog still exists and where it is apparently indigenous, in India especially, as 
as far as our experience goes, domesticated dogs when left to themselves tend rapidly to assi- 
milate in appearance to the general type of wild dog of the country, this being brought about 
partly by interbreeding with the wild dogs, and partly by degeneration of race through lack 
of supervision. In this there is a most significant peculiarity, that the interbreeding does not 
tend to assimilate the wild dog to the domestic, but the domestic to the wild ; and the domestic 
dog, though undoubtedly higher in the scale of organized beings, and with all the advantages 
of training to aid him in the struggle for existence (which should, according to the theory of 
natural selection, have made him the dominant species), is invariably extinguished. This bears 
out the idea that the domestic dog is a variety, an evidence of one phase of the law of gene- 
ration (constancy to a type), dormant for a time, reappearing with unimpaired vigour as soon 
as the external disturbing influence was removed, the inherent capacity for adaptation ceasing 
to exert itself under a recurrence to the original conditions of life. In the same way, if a 
fiock of the best-bred pigeons, containing pairs of the most diverse forms produced by selected 
breeding, be left to their own resources, they soon mix promiscuously, and in a few genera- 
tions produce an anomalous race, every individual of which resembles, except in colouring, 
and sometimes even in this, the original type of wild pigeon : it is sufiicient to illustrate the 
principle, that the anomalous forms, such as the crop of the Pouter, the tail of the Fantail 
and the rufi" of the Jacobin, do disappear when not superintended by man. We speak of 
course from a very limited experience ; but, such as it is, it bears out this view of the case, 
and, as far as we know, no variety as yet produced by artificial interbreeding and selection has 
exhibited that hereditary permanence which is so marked a characteristic of even the most 
closely allied of the natural species, such as, for instance. Fieldfares and Thrushes, Willow- 
wrens and Garden Warblers ; and this alone should make us pause before attributing to 
natural species precisely the same origin as we perceive domestic species to have had. A 
certain degree of permanence has been obtained in some varieties of domesticated animals : 
