ed are the lines of
attack of a single biological problem; for here we see, at the hands
of a few workers, a great variety of forms of life--radiates, insects,
vertebrates, low marine plants and high terrestrial ones--made to
contribute to the elucidation of various phases of one general topic,
the all-important subject of heredity. All these studies are conducted
in absolute independence, and to casual inspection they might seem to
have little affinity with one another; yet in reality they all trench
upon the same territory, and each in its own way tends to throw light
upon a topic which, in some of its phases, is of the utmost practical
importance to the human family. It is a long vault from the embryo of
an obscure sea-weed to the well-being of man, yet it may well happen--so
wide in their application are the general life principles--that study of
the one may point a practical moral for the other.

Indeed, it constantly happens that the student of biology, while
gazing through his microscope, hits upon discoveries that have the most
far-removed implications. Thus a few years ago it was discovered that
when a cell is about to bisect itself and become two cells, its nucleus
undergoes a curious transformation. Within the nuclear substance little
bodies are developed, usually threadlike in form, which take on a deep
stain, and which the biologist calls chromosomes. These chromosomes vary
in number in the cells of different animals, but the number is always
the same for any given species of animal. If one were to group animate
beings in classes according to this very fundamental quality of the
cells he would have some very curious relations established. Thus, under
the heading "creatures whose cells have twenty-four chromosomes," one
would find beings so different as "the mouse, the salamander, the trout,
and the lily," while the sixteen-chromosome group would introduce the
very startling association of the ox, the guinea-pig, the onion, and man
himself. But whatever their number, the chromosomes are always exactly
bisected before the cell divides, one-half being apportioned to each of
the two cells resulting from the division.

Now the application is this: It was the study of these odd nuclear
structures and their peculiar manouvrings that, in large measure, led
Professor Weismann to his well-known theory of heredity, according
to which the acquired traits of any being are not transmissible to the
offspring. Professor W