Fig. 4.—Nauplius larval-form of various Crustacea (Shrimps, Water-fleas, Barnacles, &c.).
The Nauplius of our shrimp gradually elongates. At first it has but three pair of limbs, but it soon acquires additional pairs, and a jointed body, and thus by gradually adding to its complexity of structure as seen in Figs. 5 and 6, it approximates more and more to the adult form from the egg of which it originated.
Fig. 5.—Larva of the Shrimp Peneus.
Fig. 6.—More advanced larva of the Shrimp Peneus.
And now we are approaching the main point to which I wish to draw the reader’s attention. In attempting to reconstruct the pedigree of the animal kingdom and so to exhibit correctly the genetic relationships of all existing forms of animals, naturalists have hitherto assumed that the process of natural selection and survival of the fittest has invariably acted so as either to improve and elaborate the structure of all the organisms subject to it, or else has left them unchanged, exactly fitted to their conditions, maintained as it were in a state of balance. It has been held that there have been some six or seven great lines of descent—main branches of the pedigree—such as that of the Vertebrates, that of the Molluscs, that of the Insects, that of the Starfish, and so on; and that along each of these lines there has been always and continuously a progress—a change in the direction of greater elaboration.
Each of these great branches of the family-tree is held to be independent—they all branch off nearly simultaneously from the main trunk like the leading branches of an oak. The animal forms constituting the series in each of these branches are supposed to gradually increase in elaboration of structure as we pass upwards from the main trunk of origin and climb further and further towards the youngest, most recent twigs. New organs have, it is supposed, been gradually developed in each series, giving their possessors greater powers, enabling them to cope more successfully with others in that struggle for existence in virtue of which these new organs have been little by little called into being. At the same time here and there along the line of march, certain forms have been supposed to have “fallen out,” to have ceased to improve, and being happily fitted to the conditions of life in which they were long ago existing, have continued down to the present day to exist in the same low, imperfect condition. It is in this way that the lowest forms of animal life at present existing are usually explained, such as the microscopic animalcules, Amœbæ and Infusoria. It is in this way that the lower or more simply-made families of higher groups have been generally regarded. The simpler living Mollusca or shellfish have been supposed necessarily to represent the original forms of the great race of Mollusca. The simpler Vertebrates have been supposed necessarily to represent the original Vertebrates. The simpler Worms have been supposed necessarily to be the stereotyped representatives of very ancient Worms.
That this is, to a certain extent, a true explanation of the existence at the present day of low forms of animals is proved by the fact that we find in very ancient strata fossil remains of animals which differ, ever so little, from particular animals existing at the present day; for instance, the Brachiopods (lamp-shells), Lingula and Terebratula, the King-crabs, and the Pearly Nautilus are found living at the present day, and are also found with no appreciable difference in very ancient strata of the earth’s crust; strata deposited so long ago that most of the forms of life at present inhabiting the earth’s surface had not then been brought into existence, whilst other most strange and varied forms occupied their place, and have now for long ages been extinct.