Page:Popular Science Monthly Volume 18.djvu/106

96 organic condition not displayed in all the vast field of life we have so far reviewed, except imperfectly, in the Cephalopod mollusks.

This is, at first, attained by the indurating of a dorsal layer of flesh into a cartilaginous cord, which stiffens the body while leaving it flexible, and furnishes points for muscular attachment.

Only a few instances remain of this earlier condition of the Vertebrate type. All others have disappeared. In the embryo of a Tunicate animal, the Ascidia, both the cartilaginous cord and the intestinal branchiæ appear. In its mature form it becomes a fixed animal, and loses this cord. But in the Appendicularia, a related animal, the cord is retained throughout life. It is also retained, in a more complete development, in the Lancelet Amphioxus, a creature having strong vertebrate affinities in its extended nerve-cord and its general functional system.

But one further step is required to produce the typical Vertebrate from such an original. This is the formation of joints in the cartilaginous cord, when it has become so firm as to resist the lateral movements of the body, or is hardened by deposition of carbonate of lime.

There is nothing in this like the welding of segments in the Articulate. The vertebrate joints display none of the separate vital animal functions. They yield every indication of being produced in the mode indicated, by the stress of an undulating body. The joints in the subsequent limbs resemble them in character, and seem to be formed in the same manner. The Lancelet is not jointed; it is a single individual. But the worm from which the Articulate arises is jointed, and each joint is possessed of all the vital functions.

Thus it appears that the Vertebrate animal starts in the race of life with advantages possessed by none of its competitors. It remains to trace the steps of its development.

 

WONDER whether it ever occurs to most people to consider how brimful our world is of life, and what a different place it would be if no living thing had ever been upon it? From the time we are born till we die, there is scarcely a waking moment of our lives in which our eyes do not rest either upon some living thing or upon things which have once been alive. Even in our rooms, the wood of our furniture and our doors could never have been if life did not exist; the paper on our walls, the carpet on our floors, the clothes on our back, the cloth upon the table, are all made of materials which life has 