X • INTEODUCTION. 
called into play, and may be fairly taken to account for many of the phenomena of Nature 
It seems also that, so far from there being a constant tendency in Nature to preserve 
fhe most divergent offspring of any one species, and thus form new species, and isolate 
the extremities by extinction of intermediate links, it is precisely these connecting links 
which in the struggle for existence should survive ; for they, being neither the one thing 
nor the other, can adapt themselves to either condition, and any cause which might tend 
to exterminate one or the other extreme would not affect to the same extent the inter- 
mediate varieties ; and they, under the complex system of laws, in ever varying conditions of 
life would remain in a perpetual state of fluctuation between the extremes, and in all cases 
be the survivors, an anomalous mixed race being the result. 
It is by the introduction of new blood that the greatest energy of variation is produced ; 
and the unmistakable and immediate effects which we perceive to arise from it in a state of 
domestication ought, if the cases were parallel, to enable us to detect it at once if it were 
to occur in a wild species ; but, notwithstanding the close watch kept over Nature in these 
days, no approach to it is discovered ; and the natural inference to be drawn is, that in a state 
of nature interbreeding of species does not take place. 
To sum up in a few words the drift of the preceding paragraphs will show the chief points 
on which the laws of modified descent, if true, are as yet obscure. 
I. The law of generation does not account for the origin of life. 
II. Animal and vegetable life, though as closely approximating in outward form as 
the internal divisions of either kingdom, are almost conclusively shown by Geology to 
have started independently. 
III. The various orders of the animal and vegetable kingdoms seem to have appeared 
too suddenly on the stage. 
IV. The absence of any proof of a general tendency in each era to approximate to 
the incoming forms. 
V. The animal and vegetable kingdoms appear to have progressed in parallel lines 
with no tendency to divergence. 
VI. The oldest stratum known to contain fossils at all contains them in great variety. 
VIL The varieties or "artificial species" produced under domestication have not 
become permanently settled in possession of their specific diagnoses. 
VIII. The unsatisfactory nature of many of the explanations of natural phenomena, 
on the hypothesis of modified descent. 
IX. The marked difference between the broods of a wild species and a domestic 
species, in the similarity outwardly of all the individuals. 
X. That an evolution theory requires the power of variation to be unlimited, whereas 
all experiments tend to show that it has a limit. 
The above considerations point to the existence in nature of causes acting in concert with 
the laws of generation, and which require to be known before the results can with any cer- 
tainty be predicted. The laws governing the animated portion of the globe are doubtless as 
definite and fixed as those that govern the motions of the great spheres of the universe ; but 
it niust bo remembered that an astronomical calculation, based accurately on the kno^^vn 
