So we see that certain plants by a change of locality, probably on account of the changed surroundings, are not as fertile as formerly. The change in temperature, the climate, in fact, has still greater influence. Here also, particularly with every change, events occur which are mostly detrimental to fertility, as is proven by the extremely numerous obserra bons of plants and animals. In the same way changes in nutrition mani fest themselves. While with a certain quality of the earth, the growth of the parts of a plant and its production of fruit remain in a proper relation to each other, the fruitfulness is very much diminished in favor of an excessive growth by an over-supply of manure. It is the same with animals, in whom over-feeding easily results in unfruitfulness. It is seldom that the opposite, an over-production, is caused. An unsuitable supply of food appears to have a similar effect, as the power of reproduc tion is diminished. A most striking influence upon the sexual capability is exerted by the change from a free state to one of subjection, as we see it in the confinement and taming of animals. The sexual powers are positively diminished; infrequent deliveries, abortions and even loss of the sexual instinct are the frequent results. But with other animals a marked increase of fertility is often brought about by taming. We also possess conclusions in regard to the influence of age upon the sexual functions, derived from observations made upon animals. From these it appears certain that advanced age as well as early youth are unfavorable to frnit fulness. This is particularly noticeable in the early youth of the mother animals. The investigations and experiments into the consequences of a sexual mixing of too closely related individuals, the so-called inbreeding, have shown an injury to the fertility in its wider sense. Much less certain is this among plants, than among aniraals, of whose descendants it may be proven that they suffer a diminution of the sexual power, in some eases even complete sterility, but most frequently there results a very unfavor able influence upon the bodily constitution. These consequences un doubtedly appear gradually in later generations. Here also belong those attempts that have been made to bring about a fruitful sexual union between two botanical or animal individuals, who, in respect to parentage and formation are widely separated. This is known as bastardizing. This latter occurs, as a rule, more readily, when two botanical or animal beings, closely related in respect to parentage, are brought into sexual union; with much more difficulty, however, the more distantly related they are. Still there are many exceptions, in which again individuals closely related will not unite so far as procreation is concerned. As far as the destiny of these newly-produced individuals, formed from a sexual union of this kind, is concerned, they are less so among plants than among animals, weaker in their reproductive power—in het they are very often absolutely unfruitful. This result finds in part its confirmation in the human species.
Exertions also have not been wanting to determine from statistics the existence of sterility. Still there are no statistics of the population of European countries that would as yet furnish us with any satisfactory explanation. Not even the frequency of sterility can be determined from the material at hand, because it is very seldom that childless marriages are investigated. But where this has been done, the material is not use ful for the determination of the absolute sterility of the marriage. For many marriages, childless at the time of investigation, may have been fruitful previouslyAnd the fruits have perished. The idea of a childless
marriage does not include an unfruitful one. Still less we gather from statistics anything that will determine for us the etiological relations of sterile marriages.
On account of the lack of statistical material, which would include great numbers of people, and would cover a great period of time, the alue of those opinions based upon the inhabitants of small territories is not to be under-estimated. By these investigations we can get a very fair idea of the frequency of unfruitful marriages; they prove that we do not go too far when we accept the proportion of sterile to fruitful mar riages as 1:8. In respect of the sexual capability, as manifested by its products, the investigations furnish us with valuable information. As to the causes of sterility, we leam from these investigations of influences whose action upon fertility is reflected in statistical tables. The influence exerted by the age of the woman at the time of marriage is particularly marked. But as useful as these statistical investigations are in themselves, they are not sufficiently far-reaching to shed a clear light upon the exist ence of sterility, and the conclusions drawn from them are not as trust worthy as they at first appear. The fact that in the statistics of M. Dun can, the influence of the sterility of the male, which furnishes a relatively large percentage of the causes of the sterility of marriage, is included, decreases their value. It is only the sterility of the married state, and not the sterility of the female, that can be explained by them. It is more probable, however, that by the term sterility, M. Duncan would not have ns understand what is usually understood by this term, but rather includes under this term conditions and occurrences which need not be in the slightest degree related to one another. According to M. Duncan almost everything that does not spe,ak for fertility, is to be regarded as an ex pression of the sterility of the female. A woman, who has repeatedly aborted or had premature births, who has repeatedly given birth to dead or misshapen fruits, is afflicted with the same cause as the woman who never conceives. But further, a woman, who repeatedly bears twins or has given birth to a dozen children, suffers from abnormal fertility, which in its nature is very closely related to sterility, or is in fact identical with it. This arbitrary collection of totally different things, which certainly does not rest upon any logical necessity, robs M. Duncan's calculations of that value which is attributed to them by the author in his investigations into the causes of sterility. In the analysis of M. Duncan's tables, the purely loeal causes of sterility are not considered, and yet both anatomical and clinical experiences have shown us that they are not as infrequent as is supposed by M. Duncan. Their influence also is accredited to more general causes.. But even if the opinion of the preponderance of general causes were positively substantiated, a neglect of the local disturbances in theory and practice would not be justified. For the.first, in order that they exert their influence, require the interposition of the latter. To cite only one instance, the sterility which, according to a law derived by M. Duncan from statistics, is very frequent in females who enter the marriage state between the ages of fifteen and twenty, can only be ex plained by those changes in the still incompletely developed genitals caused by cohabitation. It is also very probable that constitutional diseases manifest their influence in the causation of sterility only by the produc tion of local alterations.