"Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part varies more or less from the same part in the parents.... The external conditions of life, as climate and food, &c. seem to have induced some slight modifications. Habit, in producing constitutional differences, and use, in strengthening, and disuse, in weakening and diminishing organs, seem to have been more potent in their effects." And finally, as if to prevent all possible misconception, Mr. Darwin concludes his Chapter on Variation with these pregnant words:-- "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.