Page:Popular Science Monthly Volume 33.djvu/604

586 primitive than their more complex manifestations, and all analogy forces us to believe that Dysmorphosa is the descendant of some remote ancestor whose life-history was as simple and direct as that of a bird or a mammal or a frog, a butterfly, a snail, a crab, or a star-fish, and that originally each egg became converted into an adult by growth and metamorphosis.

If this is true, how has its complexity arisen? What were the stages in the gradual acquisition of the life-history which is shown in our diagram? What forces have produced the change, and what is its significance or advantage?

Not very long ago such questions were held to be unanswerable and meaningless, but at the present day we are all familiar with the process of reading the past history of life by the study of comparative anatomy and embryology, and are ready to accept the evidence of the series of living hydroids which show us the character of the changes through which the ancestors of Dysmorphosa have passed, as they have gradually acquired the structure which is exhibited by their living descendants.

I shall now briefly describe a few American species of jelly-fish, which exhibit successive stages in the process of complication, and serve to show that the remote ancestor of Dysmorphosa must have been a jelly-fish, which passed, during its development, through a transitory larval hydra stage, which was only a step in the process of growth of the embryo into an adult. In this form each egg produced one animal; the adult life was all important, and the hydra stage was passed as quickly as possible; and during the history of the species this has gradually become a more and more important part of the whole life, until finally the adult jelly-fish has become comparatively unimportant and simply serves to secure the distribution of the species, while the larvae have acquired the power to bud and to build up colonies, the members of which have become specialized in various directions, by division of labor, for the benefit of the whole.

One of the most graceful Medusæ of our Southern coast, from Florida to the Chesapeake Bay, is the beautiful Liriope shown, somewhat enlarged, in Fig. 2. It is not very different from Dysmorphosa in shape, but it is much larger, and a most active and