With respect to the inheritance of structures mutilated by injuries or altered by disease it is difficult to come to any
definite conclusion. In some cases mutilations have been practised for a vast number of generations without any inherited result. Godron has remarked[[60]] that different races of man have from time immemorial knocked out their upper incisors, cut off joints of their fingers, made holes of immense size through the lobes of their ears or through their nostrils, made deep gashes in various parts of their bodies, and there is no reason whatever to suppose that these mutilations have ever been inherited. Adhesions due to inflammation and pits from the small-pox (and formerly many consecutive generations must have been thus pitted) are not inherited. With respect to Jews, I have been assured by three medical men of the Jewish faith that circumcision, which has been practised for so many ages, has produced no inherited effect; Blumenbach, on the other hand, asserts[[61]] that in Germany Jews are often born in a condition rendering circumcision difficult, so that a name is here applied to them signifying "born circumcised." The oak and other trees must have borne galls from primeval times, yet they do not produce inherited excrescences; many other such facts could be adduced.
On the other hand, various cases have been recorded of cats, dogs, and horses, which have had their tails, legs, &c., amputated or injured, producing offspring with the same parts ill-formed; but as it is not at all rare for similar malformations to appear spontaneously, all such cases may be due to mere coincidence. Nevertheless, Dr. Prosper Lucas has given, on good authorities, such a long list of inherited injuries, that it is difficult not to believe in them. Thus, a cow that had lost a horn from an accident with consequent suppuration, produced three calves which were hornless on the same side of the head. With the horse, there seems hardly a doubt that bony exostoses on the legs, caused by too much travelling on hard roads, are inherited. Blumenbach records the case of a man who had his little finger on the right hand almost cut off, and which in consequence grew crooked, and his sons had the same finger on the same hand similarly crooked. A soldier, fifteen years before his marriage, lost his left eye from purulent ophthalmia, and his
two sons were microphthalmic on the same side.[[62]] In all such cases, if truthfully reported, in which the parent has had an organ injured on one side, and more than one child has been born with the same organ affected on the same side, the chances against mere coincidence are enormous. But perhaps the most remarkable and trustworthy fact is that given by Dr. Brown-Séquard,[[63]] namely, that many young guinea-pigs inherited an epileptic tendency from parents which had been subjected to a particular operation, inducing in the course of a few weeks a convulsive disease like epilepsy: and it should be especially noted that this eminent physiologist bred a large number of guinea-pigs from animals which had not been operated on, and not one of these manifested the epileptic tendency. On the whole, we can hardly avoid admitting, that injuries and mutilations, especially when followed by disease, or perhaps exclusively when thus followed, are occasionally inherited.
Although many congenital monstrosities are inherited, of which examples have already been given, and to which may be added the lately recorded case of the transmission during a century of hare-lip with a cleft-palate in the writer's own family,[[64]] yet other malformations are rarely or never inherited. Of these later cases, many are probably due to injuries in the womb or egg, and would come under the head of non-inherited injuries or mutilations. With plants, a long catalogue of inherited monstrosities of the most serious and diversified nature could easily be given; and with plants, there is no reason to suppose that monstrosities are caused by direct injuries to the seed or embryo.
Causes of Non-inheritance.
A large number of cases of non-inheritance are intelligible on the principle, that a strong tendency to inheritance does exist, but
that it is overborne by hostile or unfavourable conditions of life. No one would expect that our improved pigs, if forced during several generations to travel about and root in the ground for their own subsistence, would transmit, as truly as they now do, their tendency to fatten, and their short muzzles and legs. Dray-horses assuredly would not long transmit their great size and massive limbs, if compelled to live on a cold, damp mountainous region; we have indeed evidence of such deterioration in the horses which have run wild on the Falkland Islands. European dogs in India often fail to transmit their true character. Our sheep in tropical countries lose their wool in a few generations. There seems also to be a close relation between certain peculiar pastures and the inheritance of an enlarged tail in fat-tailed sheep, which form one of the most ancient breeds in the world. With plants, we have seen that the American varieties of maize lose their proper character in the course of two or three generations, when cultivated in Europe. Our cabbages, which here come so true by seed, cannot form heads in hot countries. Under changed circumstances, periodical habits of life soon fail to be transmitted, as the period of maturity in summer and winter wheat, barley, and vetches. So it is with animals; for instance, a person whose statement I can trust, procured eggs of Aylesbury ducks from that town, where they are kept in houses and are reared as early as possible for the London market; the ducks bred from these eggs in a distant part of England, hatched their first brood on January 24th, whilst common ducks, kept in the same yard and treated in the same manner, did not hatch till the end of March; and this shows that the period of hatching was inherited. But the grandchildren of these Aylesbury ducks completely lost their early habit of incubation, and hatched their eggs at the same time with the common ducks of the same place.
Many cases of non-inheritance apparently result from the conditions of life continually inducing fresh variability. We have seen that when the seeds of pears, plums, apples, &c., are sown, the seedlings generally inherit some degree of family likeness from the parent-variety. Mingled with these seedlings, a few, and sometimes many, worthless, wild-looking plants commonly appear; and their appearance may be attributed to the principle of reversion. But scarcely a single seedling will be found