ORIGIN OF SPECIES. 47 



chief deduction which appears to flow from this assuming its 

 correctness is, that there are low organisms which can exist, 

 for a certain length of time at any rate, with an extremely 

 small amount of air ; for it is to be remembered that the pro- 

 duction of a theoretically perfect vacuum is probably practically 

 impossible. If it were conceded, in fact, that a perfect vacu- 

 um had been formed in the experiments in question, the sole 

 result would be that we should have to alter all our beliefs as 

 to the conditions under which life is a possibility. The only 

 tangible result of these experiments, so far, is, that any sup- 

 posed " pre-existent germs" must have been contained, if 

 present at all, in the infinitesimal portion of air which could 

 not be expelled from the flasks experimented on ; or, they 

 must have been able to withstand without injury a tempera- 

 ture of over 212. Mr Crace-Calvert, indeed, asserts that he 

 has experimentally shown that vibrios can survive exposure to 

 a temperature exceeding 300 Fahrenheit, and Messrs Dallinger 

 and Drysdale have shown the same of the germs of bacteria. 

 Neither of these hypotheses is wholly incredible; but the ques- 

 tion ought to be regarded as still sub judice. Under any cir- 

 cumstances, the entire question is one of such complexity as to 

 be altogether unsuited for discussion here. 



i. Still more recent researches, carried out in a series of 

 elaborate experiments by Professor Tyndall, have supplied us 

 with a complete physical demonstration that ordinary atmo- 

 spheric air is invariably charged with innumerable particles of 

 solid matter, many of which are so immeasurably minute as to 

 be incapable of detection by the highest known powers of the 

 microscope. The same observer has further shown that vibrios 

 and bacteria are never produced in organic infusions, which, 

 subsequent to boiling, are exposed only to air from which these 

 floating molecules have been completely removed. In the face 

 of these observations, it is difficult to see how the doctrine of 

 " abiogenesis " can maintain its ground. 



14. ORIGIN OF SPECIES. 



It is impossible here to do more than merely indicate in the 

 briefest manner the two fundamental ideas which are at the 

 bottom of all the various theories as to the origin of species ; 

 and it will be sufficient to give an outline of the two leading 

 theories, without adducing any of the reasoning upon which 

 they are based. It should be added, however, that almost all 

 scientific men are at the present day agreed that species have 

 been produced by a process of evolution or development, 



