the adult dog. The changes which have taken countless ages in the one case, are accomplished in a few weeks in the other.

And now we have to note the important fact which makes this process of development so intensely interesting in relation to the pedigree of the animal kingdom. There is very strong reason to believe that it is a general law of transmission or inheritance, that structural characteristics appear in the growth of a young organism in the order in which those characteristics have been acquired by its ancestors. At first the egg of a dog represents (imperfectly, it is true) in form and structure the earliest ancestors of the dog; a few days later it has the form and structure of somewhat later ancestors; later still the embryo dog resembles less remote ancestors; until at last it reaches the degree of elaboration proper to its immediate forefathers.

Accordingly the phases of development or growth of the young are a brief recapitulation of the phases of form through which the ancestors of the young creature have passed. In some animals this recapitulation is more, in others it is less complete. Sometimes the changes are hurried through and disguised, but we find here and there in these histories of growth from the egg most valuable assistance in the attempt to reconstruct the genealogical tree. The history of the development of the common frog is a good illustration of the kind of evidence in question.

The frog's egg first gives rise to a little aquatic creature with external gills and a tail-the tadpolewhich gradually loses its gills and its tail and acquires in their place lungs and four legs (Fig. 2), so as now

FIG. 2.-Tadpoles and young of the Common Frog. 1, Recently hatched (twice natural size); 2 and 2a, same enlarged to show the external gills; 3 and 4, later stages with gill-slits covered by a membrane leaving only the spiracle (see Fig. 16) as an exit for the respired water; 5, with hind legs appearing; 6, with both fore and hind legs; 7, atrophy of the tail; 8, young frog.

to be fitted for life on dry land. From what we otherwise know of the structure of the frog and the animals to which it is allied, we are justified in concluding that the tadpole is a recapitulative phase of development,

and represents to us more or less closely an ancestor of the frog which was provided with gills and tail in the adult state, and possessed neither legs nor lungs.

FIG. 4.-Nauplius larval-form of various Crustacea (Shrimps, Water-fleas,

Barnacles, etc.)

A less familiar case is that of a certain kind of

shrimp, which is illustrated in the woodcuts (Fig. 3 and right lower corner of Fig. 4). The little creature

which issues from the egg of this shrimp is known as the "Nauplius form." Many animals very different in appearance from this shrimp make their first appearance in the world as Nauplii; and it appears probable that the Nauplius phase is the recapitulative representation of an ancestor common to all this set of

FIG. 5.-Larva of the Shrimp
Peneus.

FIG. 6.-More advanced larva of the Shrimp Peneus.

animals, an ancestor which was not exactly like the Nauplius, but not very different from it.

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.

And now we are approaching the main point to

In

which I wish to draw the reader's attention. 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 farther and farther 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