Popular Science Monthly/Volume 36/November 1889/Conditions Affecting the Reproductive Power in Animals
|CONDITIONS AFFECTING THE REPRODUCTIVE POWER IN ANIMALS.|
MODERN biology has made familiar the idea that animals are not fixed and unalterable in their bodily structure and functions, but, within a certain range, respond by changes in themselves to changes in their physical surroundings. This has always been observed to be true for the individual animal, as in the changes undergone in adaptation to the seasons of the year; mammals, for instance, acquiring a thicker coat of hair at the approach of winter, and reptiles and other classes passing into a low state of functional activity called hibernation or winter sleep. But it has now been well shown that this principle of modification by environment applies to species as well as to individuals. That is to say, in long periods of time species of animals are affected by changes in external conditions according to the same principle of natural law by which individual animals are affected in short periods. It is to be understood, of course, that species are affected by other causes than a changed environment—causes such as are included in Darwin's phrase of natural selection—but the fact of the modification of species becomes evident when it is seen that familiar observations made upon individual animals have an application to species also.
It is clear that the modifications thus undergone are primarily functional rather than structural, since no part of the animal body can be altered in its anatomical characters except through physiological action. But it is also true that functional modifications occur not merely as subordinate to structural changes but as ends in themselves. That is to say, functional activity may be increased or diminished in response to changes in external conditions without any necessary sequence in changes in the structure of the organs exercising the function. An illustration of this is found in the well-known fact that warm-blooded animals (excepting those that hibernate) need more food in winter than in summer to keep up their normal temperature, occasioning a considerably increased activity of the nutritive functions, but without any attendant structural changes whatever in the organs of alimentation. The same holds true with hibernating animals, which, on the other hand, take no food for a long period, the nutritive function being greatly reduced in activity, yet the organs exercising this function undergoing no structural changes. In respect to species of animals, we should not expect to find the principle hold true so strictly as in individual animals, since increased or diminished functional activity extending through many successive generations could scarcely fail to have some effect on organic structure. But the point to which special attention is here directed is that function as well as structure responds to changes of environment, and that variations in functional activity occur without any closely correlated changes in structure.
The present object is to show that the reproductive function in animals is profoundly affected by conditions of environment. It will be sufficient to state the law or principle according to which the activity of the reproductive power appears to be regulated, and then to adduce instances exemplifying the law.
When circumstances are such that most of the ova produced are likely to develop, and the young to reach maturity, then the reproductive function is least active; on the other hand, when by reason of lack of food-supply or danger of destruction by adverse physical conditions, or by natural enemies, it is probable that only a small proportion of the ova will give rise to mature animals, then the reproductive function is most active. The law may, therefore, be stated thus: The activity of the reproductive function is in proportion to the unfavorableness of the embryonic environment. The following instances are adduced:
Tape-Worm.—The common tape-worm, Tænia solium, parasitic in the human intestine, consists in structure of a series, of flat, oblong segments, sometimes eight hundred in number. Each of these segments is sexually perfect, containing both the male and female reproductive organs. The number of ova capable of development in each sexually mature segment is probably not less than five thousand. At this time the segment detaches itself from the others and is discharged from the body. In order that the ova shall develop, it is then necessary that they should gain access to the alimentary canal of the hog. If by chance they are swallowed by this animal, they quickly pass into the larval or cystic stage, burying themselves in the flesh or liver of their host, whence they may be transferred to the alimentary canal of man, where development is completed.
Now, it is obvious that this complex and fortuitous round of life renders the chances of the development of any single ovum exceedingly small. It is not probable, indeed, that one in ten thousand of the ova discharged from the alimentary canal of the host of the mature worm will ever reach the alimentary canal of the host of the larval worm. The embryonic environment is, therefore, in this case, exceedingly unfavorable by reason of its extreme narrowness. There is but one situation in which the development of the ovum can occur, and it is altogether accidental whether it reaches this situation. The explanation of the enormous capacity of the reproductive power in this animal is thus at once apparent. To compensate for the exceedingly narrow chances that the reproductive cell shall survive to complete issue, these cells are generated in excessive numbers.
Aphides.—The aphides are commonly called plant-lice, and are very abundant in summer upon the leaves of most plants. They mature quickly, at least eight or nine generations following one another in a single summer. So prolific are they that it is entirely within bounds to say that a single insect may give rise to several millions of progeny, counting the successive generations, within a few months. This astonishing fertility is dependent upon a peculiar modification of the reproductive process in these insects. During the summer there are strictly no males nor females, but all are sexually alike, and are able to produce ova which develop without fecundation, this exceptional method of reproduction being termed parthenogenesis. Here, then, we have a remarkable variation of the reproductive function resulting in an enormous increase of prolificness. And here again we find an explanation of this variation in the conditions under which the young develop. These insects are a common prey for other animals, especially birds, which devour them in great numbers, their exposed condition upon the surface of leaves rendering them easily obtainable. Hence it is that of the total number of ova produced only a very small proportion result in an increase of their kind, the young insect being devoured before completing maturation. This extraordinary and anomalous increase of the reproductive power thus furnishes an extreme instance of the operation of the law under consideration.
The Oyster.—It is well known that the oyster is very prolific. A single individual may produce over a million young. It is generally known, too, that this animal has many natural enemies, the most destructive being the star-fish. It is obvious that the reproductive power is here in relation to the unfavorableness of the natural environment, and especially during the embryonic period, when the body is small and less adequately protected, being destitute of a shell in the earliest stages.
The Codfish.—While most fishes produce eggs in great numbers, the cod is especially remarkable in this respect, a single female depositing annually eight or nine millions. The liability of destruction of the ova and young is perhaps at its maximum here, there being many natural enemies and very slight means of defense. The application of the law is obvious.
It seems unnecessary to bring forward other instances to show that the law holds good when the conditions of embryonic life are unfavorable. While it can not be so strikingly shown when the opposite conditions prevail—when the circumstances of embryonic life are favorable—it scarcely seems doubtful that it is less applicable here. When animals have abundant food-supply and ample protection against their foes and against exposure to weather, etc., the reproductive function is generally only moderately active. Without citing particular instances, it may be sufficient to point out that the largest and most intelligent animals—those that are strongest in body and quickest in instincts, and thus best able to defend themselves and their young against their enemies and to secure food and shelter—are the least prolific, bringing forth young at longer intervals and in fewer numbers.
It may have occurred to the reader that while there thus seems to be a law governing the procreative power in animals, this law is yet subordinate to another more general, more fundamental law—the law of the preservation of the species. Nature guards against the destruction of her works, and the instances of excessive activity of the reproductive function we have noted are to be interpreted as efforts made in the economy of nature to save the species from extinction.