“Whatever the cause may be of each slight difference in the offspring from their parents—­and a cause for each must exist—­it is the steady accumulation, through natural selection of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of the earth are enabled to struggle with each other, and the best adapted to survive.”

We have dwelt at length upon this subject, because of its great general importance, and because we believe that Professor Koelliker’s criticisms on this head are based upon a misapprehension of Mr. Darwin’s views—­substantially they appear to us to coincide with his own.  The other objections which Professor Koelliker enumerates and discusses are the following:[66]—­

     “1.  No transitional forms between existing species are known; and
     known varieties, whether selected or spontaneous, never go so far
     as to establish new species.”

To this Professor Koelliker appears to attach some weight.  He makes the suggestion that the short-faced tumbler pigeon may be a pathological product.

     “2.  No transitional forms of animals are met with among the organic
     remains of earlier epochs.”

Upon this, Professor Koelliker remarks that the absence of transitional forms in the fossil world, though not necessarily fatal to Darwin’s views, weakens his case.

     “3.  The struggle for existence does not take place.”

To this objection, urged by Pelzeln, Koelliker, very justly, attaches no weight.

     “4.  A tendency of organisms to give rise to useful varieties, and a
     natural selection, do not exist.

“The varieties which are found arise in consequence of manifold external influences, and it is not obvious why they all, or partially, should be particularly useful.  Each animal suffices for its own ends, is perfect of its kind, and needs no further development.  Should, however, a variety be useful and even maintain itself, there is no obvious reason why it should change any further.  The whole conception of the imperfection of organisms and the necessity of their becoming perfected is plainly the weakest side of Darwin’s Theory, and a pis aller (Nothbehelf) because Darwin could think of no other principle by which to explain the metamorphoses which, as I also believe, have occurred.”

Here again we must venture to dissent completely from Professor Koelliker’s conception of Mr. Darwin’s hypothesis.  It appears to us to be one of the many peculiar merits of that hypothesis that it involves no belief in a necessary and continual progress of organisms.

Again, Mr. Darwin, if we read him aright, assumes no special tendency of organisms to give rise to useful varieties, and knows nothing of needs of development, or necessity of perfection.  What he says is, in substance:  All organisms vary.  It is in the highest degree improbable that any given variety should have exactly the same relations to surrounding conditions as the parent stock.  In that case it is either better fitted (when the variation may be called useful), or worse fitted, to cope with them.  If better, it will tend to supplant the parent stock; if worse, it will tend to be extinguished by the parent stock.