The Whence and the Whither of Man eBook

This eBook from the Gutenberg Project consists of approximately 325 pages of information about The Whence and the Whither of Man.

The Whence and the Whither of Man eBook

This eBook from the Gutenberg Project consists of approximately 325 pages of information about The Whence and the Whither of Man.
it once lived on solid food and was therefore an animal.  For where almost the sole difference between plants and animals is in the fluid or solid character of their food, a change from the one form into the other is not as difficult or improbable as one might naturally think.  And plants and animals are here so near together, and travelling by roads so nearly parallel, that, even if volvox never was an animal, it might still serve very well to illustrate a stage through which animals must have passed.

The cells of volvox do not form a solid mass, but have arranged themselves in a single layer on the outer surface of the sphere.  For a time, under favorable circumstances, volvox reproduces very much like magosphaera, and each cell can give rise to a new, many-celled individual.  But after a time, especially under unfavorable circumstances, a new mode of reproduction appears.  Certain cells withdraw from the outer layer into the interior of the colony.  Here they are nourished by the other cells and develop into true reproductive elements, eggs and spermatozoa.  Fertilization, that is, the union of egg and spermatozoon, or mainly of their nuclei, takes place; and the fertilized egg develops into a new organism.  But the other cells, which have been all the time nourishing these, seem now to lack nutriment, strength, or vitality to give rise to a new colony.  They die.

We find thus in volvox division of labor and corresponding difference of structure or differentiation; certain cells retain the power of fusing with other corresponding cells, and thus of rejuvenescence and of giving rise to a new organism.  And these cells, forming a series through all generations, are evidently immortal like the protozoa.  Natural death cannot touch them.  These are the reproductive cells.  The other cells nourish and transport them and carry on the work of excretion and respiration.  These latter correspond practically to our whole body.  We call them somatic cells.  In volvox they are entirely subservient to, and exist for, the reproductive cells, and die when they have completed their service of these.  The body is here only a vehicle for ova.  Furthermore, in volvox there has arisen such an interdependence of cells that we can no longer speak of it as a colony.  The colony has become an individual by division of labor and the resulting differentiation in structure.

But hydra gives us but a poor idea of the coelenterata, to which kingdom it belongs.  The higher coelenterata have nearly or quite all the tissues of higher animals—­muscular, connective, glandular, etc.  And by tissues we mean groups of cells modified in form and structure for the performance of a special work or function.  The protozoa developed the cell for all time to come, the coelenterata developed the tissues which still compose our bodies.  But they had them mainly in a diffuse form.  A sort of digestive and reproductive system they did possess.  But the work of arranging these tissues and condensing them into compact organs was to be done by the next higher group, the worms.

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The Whence and the Whither of Man from Project Gutenberg. Public domain.