302 ORDER AND SUCCESSION OF LIFE. 



As is well known to every reader of modern science, 

 various hypotheses have been advanced to account for this 

 progressive development of life as revealed by geology. Pro- 

 ceeding upon the assumption that the Scheme of Vitality 

 is one united whole, these hypotheses endeavour to show 

 that all the forms of plants and animals that have appeared 

 or will yet appear are but modifications by secondary means 

 of forms that had previously existed. Admitting that the 

 origin of life is beyond the grasp of science, they neverthe- 

 less seek to explain how the lowly forms of the Primary 

 periods were developed into the higher orders of the Sec- 

 ondary ages, and these again into the still higher orders of 

 the Tertiary and Current epochs. Perceiving that plants 

 and animals do undergo modifications and variations accord- 

 ing to climate, food, and geographical position, some would 

 ascribe the whole development and ascent which Geology 

 has revealed to the mere force of external conditions, oper- 

 ating through untold ages. Others, regarding this view as 

 inadequate, would seek to combine with the force of exter- 

 nal conditions the use and disuse of organs, by which any 

 organ habitually disused may become feeble, or altogether 

 disappear, while another organ by more varied use may be 

 increased or even transformed into an organ of altogether 

 different form and fitted for an altogether different purpose. 

 By these means, the plant originally formed for the waters 

 may become fitted for the marsh, and the plant of the marsh 

 adapted for growth and reproduction on dry land. And by 

 the same process the limb originally destined to swim may 

 be transformed into one fitted for walking, and the limb 

 for walking on land converted into one adapted for flight in 

 the atmosphere. Such changes as these, they contend, must 

 take place by almost imperceptible stages, and must require 

 long ages for their elaboration the newer qualities being 

 transmitted from generation to generation, and accelerated 



