THE METEORIC HYPOTHESIS 23 



nothing whatever about them, and have as yet no pro- 

 spect 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 Metazoa 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 rind 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. Descendants of forms which in their day consti- 

 tuted the beginning of that scheme of evolution which 

 I have defined above, descendants, furthermore, of a large 

 proportion of those forms which, age after age, constituted 

 the shifting phases of its onward progress, still exist, and 

 in a sufficiently unmodified condition to enable us to recon- 

 struct, at any rate in mere outline, the history of the past. 

 Innumerable details 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 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 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 certain other descendants evolved all the 

 higher Metazoan groups. Such a process would shorten 



