Russel Wallace : Alfred Russell Wallace (sic)
The above quoted passage will seem to many readers very extraordinary if not positively erroneous. They will say: "Many diseases of man and animals are hereditary; why should not the same rule apply in plants?" And, indeed, the statement of Professor Hartig being expressly limited to the vegetable world would seem to acknowledge that it does apply in the case of animals. But although the belief that it does so apply has been till very recently almost universally held by biologists, of late years great doubt has been thrown upon the fact, due mainly to the researches of Galton and Weisman, leading to the belief that "acquired characters" are not transmissible to offspring, and that diseases are certainly in most cases acquired by the parent, not born with him.
In a very interesting and original work, "The Present Evolution of Man," by Dr. G. Archdall Reid, this subject is very fully discussed, and it is shown that, with very few exceptions, there is no proof whatever of the inheritance of disease in man, but only of the inheritance of a tendency to the special disease of the parent, so that under similar unhealthy conditions of life or of exposure to infection, the child is likely to contract the same disease, which will thus appear to be hereditary without being really so. This is clearly the case with gout and consumption, which have both been held to be hereditary, but in no case has an infant been born suffering from these diseases.
The only diseases which appear to be really transmitted are those in which a mother suffers from one of the zymotic diseases previous to the birth of her child, and the disease germs through her blood obtain access to the blood of her unborn offspring. Thus children are sometimes born apparently suffering from syphilis and even from small-pox, when the mother is, or has recently been, actually suffering from those diseases; but Dr. Reid urges that this cannot be held to prove actual heredity of the disease, but merely that the otherwise healthy child has been infected through the mother before birth, just as it might be after birth through the milk of a wet-nurse suffering from the same disease. In this latter case no one could possibly say that the infection proved the hereditary transmission of the disease, but only an infection as purely extraneous as if the poor child had been inoculated with it, or had been in close contact with another child suffering from it. It seems therefore highly probable that the statement made by Professor Hartig as regards plants is really true as regards the higher animals also; but there is a special reason why it should apply more rigidly in the case of plants which it may be as well to mention. It is very doubtful whether any of the diseases to which domesticated animals are so subject really exist among fully adult animals in a state of nature--that is, in regions where they are not in contact with domesticated animals of their own species, or where their natural conditions of life have not been injuriously modified by human agency. The cause of this immunity is the severity of the action of natural selection or the "survival of the fittest," which in this special case may be best expressed as "the extinction of the unfittest." If we consider that a wild animal can only maintain its existence day by day through being able both to obtain food and to escape from its enemies, and that any serious illness would certainly endanger its existence by rendering it unable to do either, we see that all liability to disease has been so constantly eliminated generation after generation during the whole course of the development of the species, that almost perfect health under the normal conditions of existence has long since been attained. But however rigid this selecting process is in the case of the animals, it is much more rigid in the case of most plants, because the actual or potential rate of increase is so much greater. However numerous may be the offspring of the higher animal, those of plants are far more numerous, often in the proportion of a hundred to one. Every year millions of seeds germinate which never grow into mature plants, and as the slightest tendency to disease or constitutional weakness in any seedling would certainly give that individual a special cause of extinction in addition to the general causes which affect those which are healthy, it follows that all tendency to injurious disease would be eradicated among plants even more early and more completely than in the case of animals.
Although I have no acquaintance with the detailed facts on which the statement at the head of this article was founded, I am disposed to accept it as an accurate one from its accordance with the general principles of evolution and the now generally accepted laws of heredity.