Post 00145
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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 this earth are enabled to struggle with each other, and the best adapted to survive.
Difficulties on the theory of descent with modification–Transitions–Absence or rarity of transitional varieties–Transitions in habits of life–Diversified habits in the same species–Species with habits widely different from those of their allies–Organs of extreme perfection–Means of transition–Cases of difficulty–Natura non facit saltum–Organs of small importance–Organs not in all cases absolutely perfect–The law of Unity of Type and of the Conditions of Existence embraced by the theory of Natural Selection.
Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader. Some of them are so grave that to this day I can never reflect on them without being staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to my theory.