the
Sponges, as we have them to-day, are so varied, and start from so low
a level, that no other group of animals "illustrates so strikingly the
theory of evolution," as Professor Minchin says. We begin with colonies
in which the individuals are (as in Proterospongia) irregularly
distributed in their jelly-like common bed, each animal lashing the
water, as stalked Flagellates do, and bringing the food to it. Such a
colony would be admirable food for an early carnivore, and we soon find
the protective principle making it less pleasant for the devourer. The
first stage may be--at least there are such Sponges even now--that the
common bed is strewn or sown with the cast shells of Radiolaria. However
that may be, the Sponges soon begin to absorb the silica or lime of
the sea-water, and deposit it in needles or fragments in their bed. The
deposit goes on until at last an elaborate framework of thorny, or limy,
or flinty material is constructed by the one-celled citizens. In the
higher types a system of pores or canals lets the food-bearing water
pass through, as the animals draw it in with their lashes; in the
highest types the animals come still closer together, lining the walls
of little chambers in the interior.

Here we have a very clear evolutionary transition from the solitary
microbe to a higher level, but, unfortunately, it does not take us far.
The Sponges are a side-issue, or cul de sac, from the Protozoic world,
and do not lead on to the higher. Each one-celled unit remains an
animal; it is a colony of unicellulars, not a many-celled body. We
may admire it as an instructive approach toward the formation of
a many-celled body, but we must look elsewhere for the true upward
advance.

The next stage is best illustrated in certain spherical colonies of
cells like the tiny green Volvox (now generally regarded as vegetal)
of our ponds, or Magosphoera. Here the constituent cells merge
their individuality in the common action. We have the first definite
many-celled body. It is the type to which a moving close colony of
one-celled microbes would soon come. The round surface is well adapted
for rolling or spinning along in the water, and, as each little cell
earns its own living, it must be at the surface, in contact with the
water. Thus a hollow, or fluid-filled, little sphere, like the Volvox,
is the natural connecting-link between the microbe and the many-celled
body, and may be taken to represent the first important s