24 THE AGE OF THE EARTH 



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 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 

 could only have attained to its culminating delivery 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 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 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, moreover, not a particle of evidence can be 

 brought forward. 1 



We are therefore free to follow the biological evidence 

 fearlessly. It is necessary, in the first place, to expand 



1 The arguments here set forth are only intended to oppose certain 

 rash deductions which might be drawn from Lord Kelvin's meteoric 

 hypothesis. They are in no way opposed to the hypothesis itself; still 

 less do they imply that any such conclusions were ever reached by Lord 

 Kelvin. 



