io6 MODERN IDEAS OF EVOLUTION 



unequal or disproportionate development, as the ex 

 perience of breeders of abnormal varieties of animals 

 and plants abundantly proves, imperils the continued 

 existence of the species. Changes must, therefore, 

 in order to be profitable, affect the parts of the 

 organism simultaneously and symmetrically, and 

 must be correlated with all the agencies in heaven 

 and earth that act upon the complex organism and 

 its several parts. The chances of this may well be 

 compared to the casting of aces a hundred times in 

 succession, and are so infinitely small as to be in 

 credible under any other supposition than that of 

 intelligent design. 



5. The progress of life in geological time has not 

 been uniform and uninterrupted. Just as the growth 

 of trees is promoted or arrested by the vicissitudes of 

 summer and winter, so in the course of the geological 

 history there have been periods of pause and accelera 

 tion in the work of advancement. This is in accord 

 ance with the general analogy of the operations of 

 nature, and is in no way at variance with the doctrine 

 of uniformity already referred to. Nor has it any 

 thing in common with the unfounded idea, at one 

 time entertained, of successive periods of entire de 

 struction and restoration of life. Prolific periods of 

 this kind appear in the marine invertebrates of the 

 early Cambrian, the plants and fishes of the Devonian, 

 the batrachians of the Carboniferous, the reptiles of 

 the Trias, the broad-leaved trees of the Cretaceous, 



