4O MANUAL OF ZOOLOGY. 



of the forms which are observed are the larval stages of the 

 higher Infusoria.* 



13. 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. 

 The opinions of scientific men are still divided upon this sub- 

 ject ; 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. 



I. Doctrine of Special Creation. On this doctrine of the 

 origin of species it is believed that species are immutable pro- 

 ductions, each of which has been specially created at some 

 point within the area in which we now find it, to meet the ex- 

 ternal conditions there prevailing, subsequently spreading from 

 this spot as far as the conditions of life were suitable for it. 



II. Doctrine of Development. On the other hand, it is be- 

 lieved that species are not permanent and immutable, but that 

 they " undergo modification, and that the existing forms of life 

 are the descendants by true generation of pre-existing forms " 

 (Darwin). 



* Recent researches, especially those of Dr Bastian, have established 

 some new facts as to the possibility of spontaneous generation, but they can 

 by no means be said to have settled the question, if only upon the ground 

 that they require confirmation by other experimentalists. The chief fact 

 which appears to have been established upon a tolerably firm basis is, that 

 living beings, vegetable or animal, may make their appearance in organic 

 infusions which have been subjected to a temperature of considerably over 

 the boiling-point, even though the said infusions have been hermetically 

 sealed in a flask from which all atmospheric air has been previously with- 

 drawn. The 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 production of a theoretically perfect 

 vacuum is probably practically impossible. If it were conceded, in fact, 

 that a perfect vacuum 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 supposed " 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 temperature of over 

 212. Mr Crace-Calvert, indeed, asserts that he has experimentally shown 

 that vibrios can survive exposure to temperature exceeding 300 Fahren- 

 heit. Neither of these hypotheses is wholly incredible ; but the question 

 ought to be regarded as still sub judice, and there is grave doubt as to the 

 reliability and accuracy of the experiments above alluded to. Under any 

 circumstances the entire question is one of such complexity as to be entirely 

 unsuited for discussion here. 



