CHAPTER II
It has already been pointed out, that the conditions of life are usually such as to cause the number of traits necessary for survival to be numerous, and that this again causes the process of evolution to be slow, though it is no bar to it as some biologists have supposed. But however slow the multiplicity of essential traits may render evolution, it is difficult at first sight to understand why evolution should be so extremely slow as it generally is. Every plant or animal pair usually gives origin to numerous offspring, sometimes the offspring are numbered by millions (e.g. the offspring of cod-fish); yet the number of individuals in each species does not increase as a rule, whence it is clear that nature, by destroying the majority of the offspring, exercises her power of selection with extreme stringency. Under the circumstances we might à priori expect evolution to be rapid. But what are the facts? Some types, such as the lamp-shells, have persisted almost unchanged during enormous epochs of time, and in almost all cases nature requires hundreds, nay thousands or tens of thousands of years to bring about comparatively trifling changes of structure. Why is this?
The answer is probably found if we consider certain groups of apparently unconnected facts. Children who do not especially resemble their parents often resemble their grandparents, or even more remote ancestors; at any rate they more often resemble an ancestor than
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