TRANSACTIONS OF SECTION D. 819 
to throw an undeserved suspicion upon conclusions which are sound and well 
supported, upon the firm conviction of every zoologist that the general trend of 
evolution has been, as I have stated it, that each of the Metazoan Phyla originated, 
directly or indirectly, in the Protozoa. 
The argument founded on the meteorite hypothesis would, however, require 
that the process of evolution went backward on a scale as vast as that on which 
it went forward ; that certain descendants of some central type, coming to the earth 
on a meteorite, gradually lost their Metazoan complexity and developed backward 
into the Protozoa, throwing off the lower Metazoan Phyla on the way, while cer- 
tain other descendants evolved all the higher Metazoan groups. Such a process 
would shorten the period of evolution by half, but it need hardly be said that all 
available evidence is entirely against it. 4 
The only other assumption by means of which the meteorite hypothesis might 
be used to shorten the time is even more wild and improbable. Thus it might be 
supposed that the evolution which we believe to have taken place on this earth 
really took place elsewhere—at any rate as regards all its main lines—and that 
samples of all the various phases, including the earliest and simplest, reached us 
by a regular meteoric service, which was established at some time after the com- 
pletion of the scheme of organic evolution. Hence the evidences which we study 
would point to an evolution which occurred in some unknown world with an age 
which even Professor Tait has no desire to limit. 
If these wild assumptions be rejected, there remains the supposition that, if life 
was brought by a meteorite, it was life no higher than that of the simplest Proto- 
zoou—a supposition which leaves our argument intact. The alternative supposition, 
that one or more of the Metazoan Phyla were introduced in this way while the 
others were evolved from the terrestrial Protozoa, is hardly worth consideration. 
In the first place, some evidence of a part in a common scheme of evolution is to 
be found in every Phylum. In the second place, the gain would be small; the 
arbitrary assumption would only affect the evidence of the time required for evolu- 
tion derived from the particular Phylum or Phyla of supposed meteoric origin. 
The meteoric hypothesis, then, can only affect our argument by making the 
most improbable assumptions, for which, moreover, not a particle of evidence can 
be brought forward. 
We are therefore free to follow the biological evidence fearlessly. It is neces- 
sary, in the first place, to expand somewhat the brief outline of the past history of 
the animal kingdom, which has already been given. Since the appearance of the 
‘Origin of Species,’ the zoologist, in making his classifications, has attempted as far 
as possible to set forth a genealogical arrangement. Our purpose will be served by 
an account of the main outlines of a recent classification, which has been framed 
with a due consideration for all sides of zoological research, new and old, and 
which has met with general approval. Professor Lankester divides the animal 
kingdom into two grades, the higher of which, the Enterozoa (Metazoa), were 
derived from the lower, the Plastidozoa (Protozoa). Each of these grades is again 
divided into two sub-grades, and each of these is again divided into Phyla, cor- 
responding more or less to the older sub-kingdoms. Beginning from below, the most 
primitive animals in existence are found in the seven Phyla of the lower Protozoan 
sub-grade, the Gymnomyxa. Of these unfortunately only two, the Reticularia (Fora- 
minifera) and Radiolaria, possess a structure which renders possible their preservation 
in the rocks. The lowest and simplest of these Gymnomyxa represent the starting- 
point of that scheme of organic evolution which we are considering to-day. The 
higher order of Protozoan life, the sub-grade Corticata, contains three Phyla, no one 
of which is available in the fossil state. They are, however, of great interest and 
importance to us as showing that the Protozoan type assumes a far higher organ- 
isation on its way to evolve the more advanced grade of animal life. The first- 
_ formed of these latter are contained in the two Phyla of the sub-grade Ccelentera, 
the Porifera or Sponges, and the Nematophora or Corals, Sea-anemones, 
Hydrozoa and allied groups. Both of these Phyla are plentifully represented in 
‘the fossil state. Itis considered certain that the latter of these, the Nematophora, 
