670 



SCIENCE, 



[N. S. Vol. IV. No. 97. 



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 completion of the scheme of or- 

 ganic evolution. Hence the evidences which 

 we study would point to an evolution which 

 occurred in some unknown world with an 

 age which even Prof. 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 Protozoon — a 

 supposition which leaves our argument in- 

 tact. The alternative supposition, that one 

 or more of the Metazoan Phyla were intro- 

 duced 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 evolution 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 bio- 

 logical evidence fearlessly. It is necessary, 

 in the first place, to expand somewhat the 

 brief outline of the past history of the ani- 

 mal kingdom, which has already been 

 given. Since the appearance of the ' Ori- 

 gin of Species,' the zoologist, in making his 

 classifications has attempted, as far as pos- 

 sible, 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 con- 



sideration for all sides of zoological re- 

 search, new and old, and which has met 

 with general approval. Prof. 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, corresponding more or less to the 

 older Sub-Kingdoms. Beginning from be- 

 low, the most primitive animals in exist- 

 ence are found in the seven Phyla of the 

 lower Protozoan sub-grade, the Gymno- 

 myxa. Of these unfortunately only two, 

 the Reticularia (Foraminifera) and Kadio- 

 laria, possess a structure which renders 

 possible their preservation in the rocks. 

 The lowest and simplest of these Gymno- 

 myxa 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, con- 

 tains three Phyla, no one of which is avail- 

 able in the fossil state. They are, however, 

 of great interest and importance to us as 

 showing that the Protozoan type assumes a 

 far higher organization on its way to evolve 

 the more advanced grade of animal life. 

 The first formed of these latter are con- 

 tained in the two Phyla of the sub-grade 

 Coelentera, the Porifera or Sponges, and the 

 Nematophora or Corals, Sea Anemones, Hy- 

 drozoa and allied groups. Both of these 

 Phyla are plentifully represented in the 

 fossil state. It is considered certain that 

 the latter of these, the Nematophora, gave 

 rise to the higher sub-grade, the Ccelomata, 

 or animals with a ccelom, or body-cavity, 

 surrounding the digestive tract. This lat- 

 ter includes all the remaining species of ani- 

 mal in nine Phyla, five of which are found 

 fossil — ^the Echinoderma, Gephyrea, Mol- 

 lusca, Appendiculata and Vertebrata. 



Before proceeding further, I wish to lay 

 emphasis on the immense evolutionary his- 



