The Discovery of Natural Selection 



difficult to show that a real progression in the scale of 

 organisation is perfectly consistent with all the appear- 

 ances, and even with apparent retrogression should such 

 occur. 



Using once more the analogy of a branching tree to 

 illustrate the natural arrangement of species and their 

 successive creation, he clearly shows how '^apparent re- 

 trogression may be in reality a progress, though an inter- 

 rupted one '' ; as ^* when some monarch of the forest loses 

 a limb, it may be replaced by a feeble and sickly substi- 

 tute.'' As an instance he mentions the Mollusca, which 

 at an early period had reached a high state of develop- 

 ment of forms and species, w^hile in each succeeding age 

 modified species and genera replaced the former ones 

 which had become extinct, and *^ as we approach the 

 present era but few and small representatives of the 

 group remain, while the Gasteropoda and Bivalves have 

 acquired an immense preponderance." In the long series of 

 changes the earth had undergone, the process of peopling 

 it with organic beings had been continually going on, 

 and whenever any of the higher groups had become nearly 

 or quite extinct, the lower forms which better resisted the 

 modified physical conditions served as the antetype on which 

 to found new races. In this manner alone, it was believed, 

 could the representative groups of successive periods, and 

 the risings and fallings in the scale of organisations, be in 

 every case explained. 



Again, alluding to a recent article by Prof. Forbes, he 

 points out certain inaccuracies and how they may be proved 

 to be so ; and continues : 



We have no reason for believing that the number of 

 species on the earth at any former period was much less 

 than at present; at all events the aquatic portion, with 



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