INTIiODlJOTION. 
V 
evidence of ^•e()l()f>'y t(!n(ls to sliovv Unit l,liis ailinily is 'iiol uiii vci'sal ; tin.' fjiif-st ifjii i^;, uliut, is 
the limit % 
The only way ojXMi to us for inv(\sti<^;i,tiii^- tin; niitniv) ol' tlics(; forces is 11iroii;:!j imalysis 
and comparison of the effects produced by tliem. And what are these (;fr(;f;ts ( A vast 
variety of forms of organisms of (ivery shade; and degree, and this variety of form not only 
before our eyes at the present day, but reaching backward into boundless time as far as tlie 
earth itself can carry the record — still an almost endless variety — sliowing, beyond a.ll (luf slion, 
that, as far back at least as geology can reach, whatever be the dura,tion of that pci lod, the 
organic portion of the surface of the globe, throughout its stages of progression, lias pre- 
sented a countless variety of forms. Further, as an effect, we have the fact of universal 
individual differences— that is, that no two individuals of any species can be held to be abso- 
lutely identical (we may assume this ; for it is a natural deduction from what we sec to be 
the case among the higher forms, and, as far as we have any means of testing, it has proved 
to be so with the lower) ; and these differences vary in degree and in kind. Lastly, notwith- 
standing this vast amount of variety, individuals of the same stock retain their identity to 
the f urthest limit to which we are able to trace them, and, even after a hundred generaticms, 
present no greater distinctions from their ancestors than they do from their contemporaries. 
On this we have tangible points of comparison to start from ; for the bones of the Egyptian 
mummies in one instance, and old collections of butterflies and illustrated works on ento- 
mology in another instance, cover quite as wide an interval as we have assumed above. The 
cultivation of the silkworm in China would cover thousands instead of hundreds of genera- 
tions, if any ancient pictures were extant, which is quite possible among a nation like the 
Chinese; and this may at some future time throw a light on the subject. The fact of indi- 
vidual variation is looked upon as a mainstay of the hypothesis of modified descent ; but 
how are we to define the force sufficiently powerful to nullify variation over even the short 
period we have quoted, and to control it to such an extent that each species is able to retain 
its identity, and that not a single instance of varieties of known descent becoming settled in 
possession of diff"erent diagnostic characteristics can be adduced. The ground taken that 
these individual diff"erences multiplying in the direction most advantageous to the species 
ought to produce permanent alterations, is merely a deduction. The facts before us, though 
excessively limited in extent, tend to show that they do not produce permanent alterations. 
It is inferred that species are so produced, because it is found possible to conceive that they 
might he so produced, and that if so produced the process would fit in with a grand and 
comprehensive theory : but it is impossible to avoid the admission that, as far as we know 
from actual proof positive, species are not produced by this process or by any other ; and it is 
yet unproven that new species are in the course of formation at all, though of this there are 
many possible indications. 
With regard to the behef in the immutability of species, the strongest objection seems to 
be the apparently conclusive evidence furnished by geology of the successive rise, prevalence, 
and ultimate extinction of many genera. This objection even in the present crude state of 
geological research seems formidable, and must become more so from day to day as fresh dis- 
