320 THE STORY OP THE EARTH AND MAN. 



been in operation ; and some attach more and others 

 less weight to certain of these causes, or gratuitously 

 suppose others not actually known. Of such causes 

 of change some are internal and others external to 

 the organism. With respect to the former, one 

 school assumes an innate tendency in every species to 

 change in the course of time.* Another believes in 

 exceptional births, either in the course of ordinary 

 generation or by the mode of parthenogenesis, f An- 

 other refers to the known facts of reproductive 

 acceleration or retardation observed in some humble 

 ^reatures.J New forms arising in any of these ways 

 or fortuitously, may, it is supposed, be perpetuated and 

 increased and further improved by favouring external 

 circumstances and the effort of the organism to avail 

 itself of these, or by the struggle for existence and 

 the survival of the fittest. || 



On the other hand, those who believe in the inde- 

 pendent origin of man admit the above causes as 

 adequate only to produce mere varieties, liable to 

 return into the original stock. They may either 

 hold that man has appeared as a product of special 

 and miraculous creation, or that he has been created 

 mediately by the operation of forces also concerned 

 in the production of other animals, but the precise 

 nature of which is still unknown to us ; or lastly, they 

 may hold what seems to be the view favoured by the 

 book of Genesis, that his bodily form is a product 



* Parsons, Owen. f Mivart, Ferris. 



J Hyatt and Cope. Lamarck, etc. || Darwin, etc. 



