NOVEMBEE 6, 1896.] 



SCIENCE. 



669 



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 re- 

 quired by the geologist is sufficient. 



It is first necessary to consider Lord 

 Kelvin's suggestion that life may have 

 reached the earth on a meteorite. Accept- 

 ing this view, it might be argued that 

 the evolution which took place elsewhere 

 may have been merely completed, 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 hypotiiesis on the 

 subject is that of the anxious inquirer for 

 some particle of evidence. 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 de- 

 velopment of the lowest beings with which 

 we are acquainted, from the first formed 

 beings of which we know nothing. Both 

 these processes may have required an im- 

 mensity of time ; but as we know noth- 

 ing whatever about them and have as yet 

 no prospect of acquiring any information, 

 we are 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 simplest into the most 

 complex Protozoa, the evolution of the Meta- 

 zoa from the Protozoa, and the branching 

 of the former into its numerous Phyla, with 

 all their classes, orders, families, genera 

 and species. But we shall find that this 

 is quite enough to necessitate a very large 

 increase in the time estimated by the geolo- 

 gist. 



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 con- 

 stituted the beginning of that scheme of 

 evolution which I have defined above — de- 

 scendants, furthermore, of a large propor- 

 tion of those forms which, age after age, 

 constituted the shifting phases of its on- 

 ward progress — still exist, and in a suffi- 

 ciently unmodified condition to enable us 

 to reconstruct, at any rate in mere outline, 

 the history of the past. Innumerable de- 

 tails and many phases of supreme impor- 

 tance are still hidden from us, some of them, 

 perhaps, never to be recovered. But this 

 frank admission, and the eager and prema- 

 ture attempts to expound too much, to go 

 further than the evidence permits, must 

 not be allowed to throw an undeserved sus- 

 picion upon conclusions which are sound 

 and well supported, upon the firm convic- 

 tion 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, if used to shorten 

 the time required for evolution, would, 

 however, require that the process of evo- 

 lution 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, grad- 

 ually lost their Metazoan complexity and 

 developed backward 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 



