The internal or historical causes are constant during the life-time of a species, using the term species in its most limited sense, as designating the so-called elementary species or the units out of which the ordinary species are built up. These historical causes are simply the specific characters, since in the origin of a species one or more of these must have been changed, thus producing the characters of the new type. These changes must, of course, also be due partly to internal and partly to external causes.英文原著 · 《Darwin and Modern Science》
Is it not an extraordinary fact, the great difference in position of the heart in different species of Cleodora? (33/5. A genus of Pteropods.) I am a believer that when any part, usually constant, differs considerably in different allied species that it will be found in some degree variable within the limits of the same species. Thus, I should expect that if great numbers of specimens of some of the species of Cleodora had been examined with this object in view, the position of the heart in some of the species would have been found variable. Can you aid me with any analogous facts?英文原著 · 《More Letters of Charles Darwin Volume I》
Reflect for a moment on how small and on what very peculiar causes the unequal reciprocity of fertility in the same two species must depend. Reflect on the curious case of species more fertile with foreign pollen than their own. Reflect on many cases which could be given, and shall be given in my larger book (independently of hybridity) of very slight changes of conditions causing one species to be quite sterile and not affecting a closely allied species. How profoundly ignorant we are on the intimate relation between conditions of life and impaired fertility in pure species!英文原著 · 《More Letters of Charles Darwin Volume I》
It is impossible to discuss the different results of cross-fertilisation; one point must, however, be emphasised, because Darwin attached considerable importance to it. It is inevitable that pollen of different kinds must reach the stigma. It was known that pollen of the same "species" is dominant over the pollen of another species, that, in other words, it is prepotent. Even if the pollen of the same species reaches the stigma rather later than that of another species, the latter does not effect fertilisation.英文原著 · 《Darwin and Modern Science》
The 339 species have 51 species presenting one or more varieties. The 323 species have only 37. Proportionately (339 : 323 :: 51 : 48.5) they ought to have had 48 1/2 species presenting vars. So that the case goes as I want it, but not strong enough, without it be general, for me to have much confidence in. I am quite convinced yours is the right way; I had thought of it, but should never have done it had it not been for my most fortunate conversation with you.英文原著 · 《The Life and Letters-1》
Lyell, indeed, has spoken of an hypothesis that 'the successive creation of species may constitute a regular part of the economy of nature,' but he has nowhere, I think, so described this process as to make it appear in what department of science we are to place the hypothesis. Are these new species created by the production, at long intervals, of an offspring different in species from the parents? Or are the species so created produced without parents? Are they gradually evolved from some embryo substance? Or do they suddenly start from the ground, as in the creation of the poet?...英文原著 · 《The Life and Letters-1》
There are 1.7 million known species on the planet, and biologists estimate this represents only a quarter of the total species on the planet.美剧 · 《The News Room》
Species diversity is greatest in rain forests, and 75% of all species live in rain forests.美剧 · 《The News Room》
Sometimes you catch yourself watching all the people like they're another species.美剧 · 《Billions》