The change may be only in the time of flowering in plants, or season of heat in animals, or it may be actual infertility in sexual union.
Since 1866 the agricultural inquiry has demonstrated the invasion of Malthusianism and the progress of voluntary infertility in almost every department side by side with the subdivision of the soil.
It did not convince Darwin then, but it has convinced me now, and I think it can be proved that in some cases (and those I think most probable) Natural Selection will accumulate variations in infertility between incipient species.
You urge the improbability of the required infertility being correlated with the particular variations which characterised each incipient species.
Many other causes of infertility co-operate, and I really think I have overcome the fundamental difficulties of the question and made it a good deal clearer than Darwin left it.
It may be that such lack of harmony is the ground of the relative infertility of blended stocks.
From this study and his own experience, he concludes that close interbreeding is necessary to ennoble the stock; but that in effecting this the greatest care is necessary, on account of the tendency to infertility and weakness.
But this cause has no bearing on the infertility of first unions.
Considering that infertility and prenatal mortality eliminate approximately 40 per cent of eggs laid (according to laboratory findings) the average number of surviving young per clutch would be 2.
Eggs laid in the laboratory had higher rates of infertility and prenatal mortality than did eggs dissected from oviducts.
Infertility and prenatal mortality account for at least 40 per cent of the eggs laid, according to laboratory findings.
Hence, as fixed differences of form and colour, slowly gained by natural selection in adaptation to changed conditions, are what essentially characterise distinct species, some amount of infertility between species is the usual result.
The doctrine of the infertility of hybrids seems to have been founded on the fact that the hybrids best known to breeders, namely the cross between the ass and the horse, and those between the canary and other finches, are sterile.
He has boldly tried to make an ally of this bogey of the infertility of hybrids.
The striking differences between the races are first emphasised, and the question of the fertility or infertility of hybrids is discussed.
Hence the very remarkable contrast between our domesticated varieties and natural species with regard to cross-infertility is just what the present theory would expect, or, indeed, require.
Hence, if cross-infertility has thus usually been a necessary condition to a varietal divergence becoming specific, it can make no material difference when the incipient infertility arose.
This refers to what I understand Mr. Wallace to say in the Nature correspondence is the supposition on which his own theory of the origin of species by cross-infertility is founded.
Can you doubt, after duly considering the facts of prepotency on the one hand and those of Jordan's physiological varieties on the other, that cross-infertility does arise before or during the specific differentiation?
In this connection also should be considered the infertility and other evils resulting from the intermarriage of near kindred.
Among domestic animals close interbreeding, it is well known, leads to infertility and degeneration; and Darwin's researches prove that self-fertilization in the vegetable kingdom produces the same results.
But theinfertility of polyandrous families is unfavorable to their survival, for there are fewer members available for defense.
But if in the nature of things absolute sterility is not inheritable, comparative infertility may be.
The infertility of the best stock is due to voluntary curtailment of the family, through sexual self-restraint.
This is the infertility of the best variety, that of the "Cheribon" cane.
In the case of Oenothera muricata x biennis the differentiating units reduce the fertility to a low degree, threatening the offspring with almost complete infertility and extinction.
Eugenically, then, we should ask whether in human society there is at present any such association of superfertility or infertility with desirable or undesirable traits.
This is apparently hereditary, but before that could be asserted definitely to be so here or in any similar case, we should know that the infertility were not the result of an infection to which immunity is rare or unknown.
This fact has been found true also for still less analyzable characters such as length of life, fertility or infertility and the like, and again about the same intensity of resemblance is found.
Or if infertility is so associated, then the average of the whole group moves away from that condition.
In order to avoid over- estimating the infertility of the several illegitimate unions, I have taken the normal standard as low as possible.
There is another point which deserves notice, namely, the relative degree of infertility in the several species of the long-styled and short-styled flowers, when both are illegitimately fertilised.
The degree ofinfertility of a cross is liable to much fluctuation.
Large capsules were always selected for counting, in order to avoid over-estimating the infertility of the several illegitimate plants.
The infertility of the women is even a more remarkable fact than the liability of all to ill-health and death.
The scheme also assumes either selective fertilization or, what amounts to the same thing, infertility of gametic unions where like sex chromosomes are present.
Most of our domestic breeds have been so often crossed, and their mongrels so largely kept, that it is almost certain, if any degree of infertility had existed between them, it would have been detected.
Mr. Sclater on the infertility of animals under confinement.
