single continuous entity. One of these livings things was
unhesitatingly regarded as being in all respects a unit. Parts it might
have, various in their sizes, forms, and compositions; but these were
components of a whole which had been from the beginning in its original
nature a whole. Even to naturalists fifty years ago, the assertion that
a cabbage or a cow, though in one sense a whole, is in another sense a
vast society of minute individuals, severally living in greater or less
degrees, and some of them maintaining their independent lives
unrestrained, would have seemed an absurdity. But this truth which, like
so many of the truths established by science, is contrary to that common
sense in which most people have so much confidence, has been gradually
growing clear since the days when Leeuwenhoeck and his contemporaries
began to examine through lenses the minute structures of common plants
and animals. Each improvement in the microscope, while it has widened
our knowledge of those minute forms of life described above, has
revealed further evidence of the fact that all the larger forms of life
consist of units severally allied in their fundamental traits to these
minute forms of life. Though, as formulated by Schwann and Schleiden,
the cell-doctrine has undergone qualifications of statement; yet the
qualifications have not been such as to militate against the general
proposition that organisms visible to the naked eye, are severally
compounded of invisible organisms--using that word in its most
comprehensive sense. And then, when the development of any animal is
traced, it is found that having been primarily a nucleated cell, and
having afterwards become by spontaneous fission a cluster of nucleated
cells, it goes on through successive stages to form out of such cells,
ever multiplying and modifying in various ways, the several tissues and
organs composing the adult.

On the hypothesis of evolution this universal trait has to be accepted
not as a fact that is strange but unmeaning. It has to be accepted as
evidence that all the visible forms of life have arisen by union of the
invisible forms; which, instead of flying apart when they divided,
remained together. Various intermediate stages are known. Among plants,
those of the _Volvox_ type show us the component protophytes so feebly
combined that they severally carry on their lives with no appreciable
subordination to the life of the group. And among animals, a paral