ORIGIN OP SPECIES. 35 



is perfectly well known, and it consists either in a true sexual 

 process, for which proper organs are provided (as in Paramce- 

 cium), or in a process of gemmation or fission. It is, there- 

 fore, contrary to analogy to suppose that they should be gene- 

 rated in the manner maintained by the heterogenists, since 

 this mode of reproduction would appear to be superfluous. 



li. In the absence of any direct proof to the contrary, it is 

 safer to adopt an explanation of the observed phenomena 

 which does not have recourse to laws with which we are as 

 yet unacquainted. Thus, it is not at variance with any known 

 law to suppose that the primary molecules are the result of 

 the development of germs which find in the organic infusion 

 a suitable nidus ; that these primary molecules and the vibrios 

 which they produce are referable to the Protophyta, and 

 should probably be placed near the filamentous Confervas ; 

 that by the death of these vegetable organisms the fluid is 

 prepared for the reception and development of the germs of 

 the Protozoa, for which the former serve as pabulum ; and 

 that many 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 

 external 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 a-nd immutable, but that 

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

 are the descendants by true generation of pre-existing forms.' 



,rwin.) 



n Lamarck's theory of the development of species, the 

 s of modification were ascribed to the action of external 

 sical agencies, the inter-breeding of already existing forms, 

 the effects of habit. 

 The doctrine of the development of species by variation and 

 natural selection propounded by Darwin, and commonly 



D 2 





