The instances just surveyed, these extremes of the potentiality of procreation, are instructive in more ways than one. They are to be regarded as “excrescences” of reproduction, comparable to those “excrescences” of individual growth which we call “ornament,” for example. Individuals on whom this fertility has settled, so to speak, are the victims of the machinery of sex and reproduction. Their amazing powers of multiplication are not of their own seeking, they are inherent manifestations of variations of growth, uncontrollable save by the machinery of Natural Selection. Incidentally such victims serve a useful purpose, for their myriad hosts afford food for hordes of other animals, which in turn are eaten. Little though we realize it, the well-being of the human race would suffer if these prolific creatures—the uncomplaining victims of that inexorable law which bids all living things “increase and multiply” or die—should cease to be; for with them would disappear a host of animals on whose existence man’s comfort more or less depends.
During the millions of years that have rolled by since the first appearance of life on the earth, who shall count the number of types which have been exterminated without leaving the faintest trace of their having ever existed? The survivors which have contrived to maintain a place in the sun present an infinite range of variation in colour, size, habit, and structure, as well as in emotions. These varied aspects are all so many facets of the mysterious phenomenon we call Life: and they are so many witnesses of the versatility of Life. Not the least mysterious feature of this Life is its faculty of reproduction, which expresses itself in an infinite variety of ways, defying all but the crudest forms of analysis. The evolution of sex has exercised the speculative ingenuity of some of the acutest students of Nature from the earliest times, and we are still far from a satisfactory solution of the problems it presents. Hermaphroditism and Parthenogenesis are commonly regarded as degenerate forms of reproduction, but it would probably be more correct to see in them exceptional modes of adaptation enabling such individuals to occupy niches in the world untenable to creatures of more conservative habit. That the peculiar “strains” of animal life have turned into backwaters which offer no opportunity or possibility of further advancement seems clear enough, but they are nevertheless interesting and instructive.
The parthenogenetic Crustacea and the Rotifers afford some good evidence of this adaptability—of the way in which creatures manage to cling to the skirts of life by reason of their power to survive the extremest tests of endurance. And this success has largely been due to some mysterious property of the germ-plasm enabling reproduction to take place through the female line alone, or in some cases with an occasional fillip from the intervention of males. Of the many marvellous things that could be related of these creatures but few instances can be cited here.
The case of the Brine Shrimp (Artemia salina) will afford an exceptionally good illustration because the facts can be tested by anyone who will take the trouble to make a simple experiment for himself. Those anxious to do this should dissolve eight ounces of Tidman’s sea-salt in a glass jar containing five pints of water, keeping the mixture well stirred till the salt is dissolved. It should be allowed to stand and be carefully watched. In about three days, with a pocket-lens, or even without, minute white specks will be seen moving with a jerky motion up and down the water. These are larval Brine Shrimps. Now they must be fed. Take a piece of lettuce-leaf or any green stuff, and pound it up, or grind it up with a knife-blade on a plate with a little water, till the whole is reduced to the consistency of green paint; then empty this into the water. This must be done daily, or at any rate frequently. Quickly these tiny specks will grow into Brine Shrimps, translucent creatures nearly half an inch long, swimming about back downwards with a marvellously rhythmical movement of delicate feet. In all probability no males will be found, but, on the other hand, both sexes in almost equal numbers may be present. The males may readily be distinguished by their massive arms immediately behind the head, for the purpose of embracing the females.
Whence came these wonderful animals? The mystery is easily explained. The salt is genuine sea-salt, formed in brine-pans, chiefly in the Mediterranean. As the water evaporated the Shrimps it contained gradually died; but the eggs in the females became encapsuled in the salt-crystals to hatch out long months after. In one of my own experiments I succeeded with salt that I had kept for more than a year. Of course, every sample of salt experimented with will not yield successful results, but failures are not expensive. Now in this brine-pan there were myriads of other animals which were killed outright: the Brine Shrimp is at least able to pass on descendants by reason of the vitality of its eggs. Some near relations of the Brine Shrimps live in fresh water and possess similar powers of resistance to adverse conditions. The Fairy Shrimp (Chirocephalus) is one of these. Not unlike its cousin the Brine Shrimp in appearance, it lives in shallow pools, such as have muddy bottoms and are constantly liable to dry up. Birds hunting by the margins of the pool where the retreating water has left a fringe of mud bear away more or less of this on their feet and transport it to similar pools, or even puddles. Such transplanted samples may easily contain numbers of eggs of this tiny creature. Only a year or two ago Fairy Shrimps were found in abundance in rain pools at Eton, and some, indeed, were discovered swimming gaily about in a rain-filled cart-rut!
Another very singular Crustacean, known as Apus, bears a curious superficial likeness to the King Crab (Limulus), having a large back-shield and a long tail. This little creature, a giant compared with his nearest relations, is an inhabitant of wayside ponds and ditches. Thousands of females may be taken for years in succession without the advent of a single male. Then, for some strange reason which we cannot even guess at, males appear. Like its freshwater cousin, the Fairy Shrimp, Apus can withstand drought: its favourite haunts may be transformed into sun-baked hollows, but with a heavy fall of rain and a few hours’ soaking the eggs left by dead females develop, and once more the pool and its inhabitants are established again. Having regard to the extraordinary vitality of these small creatures, it is curious that they should ever disappear from their favoured haunts. But they do. Not many years ago Apus could be found in abundance in many parts of the South of England. It is now extinct; its last resorts were the ponds at Hampstead: now one may search in vain for them. “No British specimens,” remarks Dr. Caiman, a great authority on the Crustacea, “had been recorded for over forty years, and the species was believed to be extinct in this country, when it was found in 1907 by Mr. F. Balfour Browne in a brackish marsh near Southwick, in Kirkcudbrightshire.” These had probably developed from eggs accidentally transported by some bird from the Continent. The extinction of the race throughout the British Islands can only be attributed to the too long absence of males, and the consequent inability to restore vigour by the more normal method of reproduction by sexual congress.
Among the Rotifers the little Wheel-animalcules exhibit an even greater vitality, for not only can their eggs withstand prolonged desiccation, but in some the body of the animal survives even harsher treatment. If specimens be enclosed within a chamber containing a little sand or moss the contents may be dried over sulphuric acid, or heated up to 200° F., or left to the neglected dust of years, and will yet revive if a little fresh water be added to the sand. Males are rare, and when they do occur are little more than animated receptacles for semen, for they are incapable of feeding, the gullet and digestive tract being reduced to a solid cord. A certain amount of nourishment, however, may be absorbed through the delicate body wall.
The degeneration of the males in these parthenogenetic species irresistibly reminds one of the smile of the Cheshire cat; they grow smaller and smaller, and their functions less and less, till finally nothing is left. The “complemental males” discovered years ago by Darwin in the Barnacles well illustrate this process. In dissecting adult specimens of the stalked Barnacle (Scalpellum) he found, just inside the valves, in a pocket of the mantle, a varying number of “complemental males,” tiny organisms which Mr. Geoffrey Smith describes as “little more than bags of spermatozoa,” and they apparently serve to fertilize the ripe ova of the larger animal—one cannot say of the female, for Scalpellum, like most of the Barnacles, is hermaphrodite. But it is believed that these complemental males are really arrested hermaphrodites. At any rate, if it so be noted that with some of the Barnacles, as with some other Crustacea, the larvæ are males, but when adult life is attained female glands appear and hermaphroditism is established. Such hermaphrodites have the singular distinction of being males which have acquired female attributes, true females being unknown among them!
In one of the parasitic Crustacea (Chondracanthus) infesting the gills of Gurnard, Plaice, Skate and other fish, the adult female is about half an inch long, and very unlike a Crustacean in appearance; the male is an extremely minute maggot-like object—a few millimetres in length—and lives permanently attached to the belly of his mate just at the base of the egg masses. More remarkable still is the case of another nearly related parasitic species—Lernea—which becomes sexually mature in its childhood. The males perform their part and die; their mates arrive at maturity and settle down to a comfortable life as parasites on fish, reproducing without further mating.
That Parthenogenesis and Hermaphroditism are but specialized forms of reproduction, leading sooner or later to degeneration and extinction, there can be no doubt. They are, so to speak, failures in the evolution of sex, demonstrating in a very forcible fashion the impossibility of progress—as we understand it—where the sexual functions are thus combined.