September 24, 1896] 



NA TURE 



505 



comparatively shallow water and near coasts, are only founil in 

 certain beds, ami then in far less abundance than in the oceanic 

 deposit. We can only explain this difference by supposing that 

 the latter accumulate with such almost infinite slowness as 

 compared with the continental deposits that these remains form 

 an important and conspicuous constituent of the one, while they 

 are merely found here and there when looked for embedded in 

 the other. The rate of accumulation of all other constituents is 

 so slow as to leave a layer of teeih and ear-bones uncovered, or 

 covered by so thin a deposit that the dredge can collect them 

 freely. Dr. John Murray calculates that only a few inches of 

 this deposit have accumulated since the Tertiary Period. These 

 most interesting facts prove furthermore that the great ocean 

 basins and continental areas have occupied the same relative 

 positions since the formation of the first stratified rocks ; for no 

 oceanic deposits are found anywhere in the latter. We know 

 the sources of the oceanic deposit, and it might be possible 

 to form an estimate, within wide limits, of its rate of accumu- 

 lation. If it were possible to ascertain its thickness by means 

 of a boring, some conclusions as to the time which lias 

 elapsed during the lifetime of certain species — perhaps even 

 the lifetime of the oceans themselves — might be arrived at. 

 Lower down the remains of earlier species would probably be 

 found. The depth of this deposit and its character at deeper levels 

 are"questions of overwhelming interest ; and perhaps even-more 

 so is the question as to what lies beneath. Long before the 

 C/iai/tiixir had proved the persistence of oceanic and continental 

 areas, Darwin, with extraordinary foresight, and opposed by all 

 other naturalists and geologists, including his revered teacher, 

 Lyell, had come to the same conclusion. His reasoning on the 

 subject is so convincing that it is remarkable that he made so 

 few converts, and this is all the more surprising since the argu- 

 ments were published in the "Origin of Species," which in 

 other respects produced so profound an effect. In speculating as 

 to the rocks in which the remains of the ancestors of the earliest 

 known fossils may still exist, he suggested that, although the 

 existing relationship between the positions of our present oceans 

 anil continental areas is of immense antiquity, there is no reason 

 for the belief that it has persisted for an indefinite period, but 

 that at some time long antecedent to the earliest known fossil- 

 iferous rocks "continents may have existed where oceans are 

 now spread out ; and clear and open oceans may have existed 

 where our continents now stand." Not the least interesting 

 result would be the test of this hypothesis, which would probably 

 be forthcoming as the result of boring into the floor of a deep 

 ocean ; for although, as Darwin pointed out, it is likely enough 

 that such rocks would be highly metamorphosed, yet it might 

 still be possible to ascertain whether they had at any lime formed 

 part of a continental deposit, and perhaps to discover much 

 more than this. Such an undertaking might be carried out in 

 conjunction with other investigations of the highest interest, such 

 as the attempt to obtain a record of the swing of a pendulum at 

 the bottom of the ocean. 



We now come to the strictly biological part of our subject — 

 to the inquiry as to how much of the whole scheme of organic 

 evolution has been worked out in the time during which the 

 fossiliferous rocks were formed, and how far, therefore, the time 

 required by the geologist is sufficient. 



It is first necessary to consider Lord Kelvin's attempt to 

 rescue us from the dilemma in which we were placed by the in- 

 sufficiency of time for evolution — the suggestion that life may 

 have reached the earth on a meteorite. According to this 

 view, the evolution w hich took place elsewhere may have been 

 merely comjileted, in a comparatively brief space of time, on 

 our earth. 



We know nothing of the origin of life here or elsewhere, and 

 our only attitude towards this or any other hypothesis on the 

 subject is that of the anxious inquirer for some particle of evi- 

 dence. But a few brief considerations will show that no escape 

 from the demands for time can be gained in this way. 



Our argument does not deal with the time required for the 

 origin of life, or for the development of the lowest beings with 

 which we are acquainted from the first formed beings, of which 

 we know nothing. Both these jirocesses may have required 

 an immensity of time ; but as we know nothing whatever about 

 them, and have as yet no prospect of acquiring any information, 

 we arc compelled to confine ourselves to as much of the process 

 of evolution as we can infer from the structure of living and 

 fossil forms — that is, as regards animals, to the development of 



the siui|ilest into the most complex Protozoa, the evolution of 

 the Melazoa from the Protozoa, and the branching of the former 

 into its numerous Phyla, with all their Classes, Orders, Families, 

 Genera, and Species. But \vc shall find that this is quite 

 enough to necessitate a very large increase in the time estimated 

 by the geologist. 



The Protozoa, simple and complex, still exist upon the earth 

 in countless species, together with the Metazoan Phyla. De- 

 .scendants of forms which in their day constituted the beginning 

 of that scheme of evolution which I have defined above, de- 

 scendants, furthermore, of a large proportion of those forms 

 which, age after age, constituted the shifting phases of its on- 

 ward ])rogres.s, still exist, and in a sufficiently unmodified con- 

 dition to enable us to reconstruct, at any rate in mere outline, 

 the history of the past. Innumerable details and many phases 

 of supreme importance are still hidden from us, some of them 

 perhaps never to be recovered. But this frank admission, and 

 the eager and premature attempts to expound too much, to go 

 further than the evidence permits, must not be allowed 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 meteorite theory 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 Ijaclcward into the Protozoa, throwing 

 off the lower Metazoan Phyla on the way, while certain 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. 

 The only other assumption by means of which the meteorite 

 hypothesis would serve 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 cjf all the various jih.ases, including the earliest and 

 simplest, reached us by a regular meteoric service, which was 

 estalilished at some time after the completion 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 Prof. Tait has no desire to limit. 



If these wild assumptions be rejected, there remains the sup- 

 position 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 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 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, more- 

 over, not a particle of evidence can be brought forward. 



We are therefore free to follow the biological evidence fear- 

 lessly. It is necessary, 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 lie 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. Prof. 

 Lanke.ster divides the animal kingdom into two grades, the 

 higher of which, the Enterozoa (Metazoa), were derived froin 

 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 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 (Foraminifera) and Radiolaria, possess 

 a structure which renders possible their preservation in the 



NO. 1404, VOL. 54] 



