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which are not parts of the living substance of the organism, but which simply owe their existence to the properties of its Hving substance, we can readily understand that the complexity of an adult animal may be vastly greater than the complexity of the egg.
In the second place we must recollect that there are many race characteristics which are of constant occurrence without being hereditary.
Organisms are often greatly modified by the direct action of external conditions; for instance, a tree may be dwarfed by insufficient food, or the muscles of a limb may be greatly enlarged by unusual work. If all the individuals of a species are similarly exposed to conditions of this sort, they will all be acted upon in the same way, and the modification which is thus produced will be characteristic of the species, without being hereditary.
To take one of the simplest cases: Trees which grow upon mountain-tops, where they are exposed to extreme changes of climate, and to constant and violent winds, hare a very characteristic appearance, which is familiar to all mountain climbers. In some cases this peculiar form is hereditary, and persists in seedlings which are grown in more favored regions, but in other species the transplanted trees show, by losing their peculiarities, that these are due to direct modification.
If a certain species occurs naturally nowhere except in such situations, this species will be characterized by its dwarfed size and by its twisted and distorted branches; but if individuals reared under favorable influences grow and flourish and become regular and symmetrical, we may conclude that the characteristics of each wild individual are caused by its scanty food and constant exposure, and that they are not represented in the egg, and are not congenital.
If this experiment is impossible, if all the^ransplanted trees die, and if the seeds fail to germinate in fertile ground, there will be no way to show whether the peculiar characteristics of the species are or are not hereditary.
We know that organisms may be modified in many ways by the direct action of external conditions, but a few illustrations will not be out of place.
Hemp-seed causes bullfinches and certain other birds to become black, and we know from the observations of many naturalists that caterpillars which are fed on different kinds of food either themselves acquire a different color, or they may produce moths which differ in color. Many curious cases of this kind have been noticed in birds and insects, and if unnatural food causes deviations from the natural color of a species, it is quite possible that the normal color may in many cases be due directly to the action of the normal or natural food.
Darwin gives marly instances of plants which are characterized by a certain peculiarity in one country, while in another country this peculiarity is almost or entirely lacking. Thus when the American sassafras tree is grown in Europe, it loses its aromatic flavor. In India the fibres of flax and hemp are brittle and useless, and the latter plant yields a resinous narcotic substance, hasheesh, which is used as an intoxicating drug, but in England this property is lost and the fibre becomes long and tough. Large, finely-flavored, and brightly-colored American apples, when reared in England, produce fruit of a dull color and poor quality.
In these cases we are unable to state what the determining conditions are, but the fact that peculiarities are made to disappear by a change from one country to another shows that they are not congenital but are due to something outside the plant, which is present in one country but absent in another. The following instance, which*is given by Darwin, is most interesting: '.' Mr. Salter, who is well known for his success in cultivating variegated plants, informs me that rows of strawberries were planted in his garden in 1859, in the usual way; and at various distances in one row several plants simultaneously became variegated, and what made the case more extraordinary, all were variegated in precisely the same manner. These plants were removed, but during the three succeeding years other plants in the same row became variegated, and in no instance were the plants in any adjoining row affected." He also says that in certain parts of India the turkey becomes reduced in size with the pendulous appendages over the head enormously developed.
In these cases it is difficult to determine what has caused the change, but in other instances this is more obvious. Thus Darwin states that good authorities assert that horses kept during several years in the deep coal mines of Belgium become covered with velvety hair almost like that of the mule, and he quotes from Dr. Falconer the statement that the Thibet mastiff and goat when brought down from the Himalayas to Kashmir lose their fine wool.
These are only a few of the cases which Darwin gives, and many more might be added from other authorities, but I have given enough to show that external conditions of life may act in one country to cause certain modifications which are entirely absent in another country.
The change of Artemia into Branchippus, by rearing it in fresh water, is one of the most remarkable instances of definite modification due to a change of external conditions. Artemia salina is a small crustacean, found in the salt lakes of America, Europe, and Africa. When this species is kept in water in which the quantity of salt is gradually diminished, it becomes transformed, in a few generations, into what has been described as a distinct species—Artemia Milhausenii—and if the process of diluting with fresh water is continued until it finally becomes perfectly fresh, the Artemia becomes changed into the well-known fresh-water form Branchippus, which has always been considered a distinct genus.
Semper has shown (Animal Life, p. 161) that certain definite changes in the size of the fresh-water snail Lymnsea are produced in a short time by confining it in a small quantity of water.
These are a few of the cases where we are able to show, by experiment, that certain race-characteristics are not congenital, but are due to external influences, and we have every reason to believe that the same thing is true in many cases which have never been made the subject of experiment, and in many more where experiment is impossible, since the change would cause death rather than modification.
The possibility that structures of the greatest constancy and importance may not really be hereditary is well illustrated by Hunter's well-known experiments on the sea-gull. In pigeons, and in most birds which feed upon grain, the muscular wall of a portion of the stomach is greatly developed, to form the crushing and grinding gizzard, which is lined with a covering of tough membrane, while the stomach of the gull and of most fleshfeeding birds is soft, and the muscular layer little developed. Hunter fed a sea-gull for a year on grain, and he thus succeeded in hardening the inner coat of the bird's stomacb, thus forming a true gizzard; and Darwin quotes from Dr. Edmonston the statement that a similar change occurs twice a year in the stomach of another sea-gtill in the Shetland Islands, where this bird frequents the corn-fields and feeds on seeds in the spring, but catches fish during the rest of the year. This observer has noticed a great change in the stomach of a wren which had long been fed on vegetable food; and Menetries states that when an owl was similarly treated the form of the stomach was changed, and the inner coat became leathery, while the liver increased in size. Semper states that Dr. Holmgrin has been able to transform the gizzard of a pigeon into a carnivorous stomach by feeding the bird on meat for a long time.
There is no reason for believing that the few cases known to us are all which are due to the direct action of external conditions, and we must acknowledge that there may possibly be many structural characteristics of animals and plants which are not hereditary, but are constant simply because the conditions which cause them are constant, and as we are only compelled to attribute to the ovum representatives of all the hereditary race characteristics, it will be seen that the structural complexity of the egg may be vastly less than that of the developed organism.
This is not all, however. There may be many congenital race characteristics which are not hereditary.
The various parts of a developing organism are exposed in countless ways to the influence of other parts. The simplest illustration of this fact is the mechanical pressure exerted upon each other by the developing viscera.
This is a subject which is almost outside the province of experiment, for we cannot shut out the influence of any particular organ without removing the organ itself, and the removal of any organ of considerable size is