192 Mr. A. R. Wallace on the Law which has regulated 



modified species and genera replaced the former ones which had 

 become extinct, and as we approach the present sera but few 

 and small representatives of tlie group remain, while the Gaste- 

 ropods and Bivalves have acquired an immense preponderance. 

 In the long series of changes the earth has undergone, the 

 process of peopling it with organic beings has been continually 

 going on, and whenever any of the higher groups have become 

 nearly or quite extinct, the lower forms which have better 

 resisted the modified physical conditions have served as the 

 antitypes on which to found the new races. In this manner 

 alone, it is believed, can the representative groups at successive 

 periods, and the risings and fallings in the scale of organization, 

 be in every case explained. 



The hypothesis of polarity, recently put forward by Professor 

 Edward Forbes* to account for the abundance of generic forms 

 at a very early period and at present, while in the intermediate 

 epochs there is a gradual diminution and impoverishment, till 

 the minimum occurred at the confines of the Palaeozoic and 

 Secondary epochs, appears to us quite unnecessary, as the facts 

 may be readily accounted for on the principles already laid 

 down. Between the Palaeozoic and Neozoic periods of Professor 

 Forbes, there is scarcely a species in common, and the greater 

 part of the genera and families also disappear to be replaced by 

 new ones. It is almost universally admitted that such a change 

 in the organic w-orld must have occupied a vast period of time. 

 Of this interval we have no record ; probably because the whole 

 area of the early formations now exposed to our researches was 

 elevated at the end of the Palaeozoic period, and remained so 

 through the interval required for the organic changes which 

 resulted in the fauna and flora of the Secondary period. The 

 records of this interval are buried beneath the ocean which 

 covers three-fourths of the globe. Now it appears highly pro- 

 bable that a long period of quiescence or stability in the physical 

 conditions of a district would be most favourable to the existence 

 of organic life in the greatest abundance, both as regards indi- 

 viduals and also as to variety of species and generic groups, just 

 as we now find that the places best adapted to the rapid growth 

 and increase of individuals also contain the greatest profusion 

 of species and the greatest variety of forms, — the tropics in 

 comparison with the temperate and arctic regions. On the 

 other hand, it seems no less probable that a change in the 



* Since the above was written, the author has heard with sincere regret 

 of the death of this eminent naturahst, from uliom so much important 

 work was expected. His remarks on the present j)aper, — a subject on 

 M hieh no man was more competent to decide, — were looked for with the 

 greatest interest. Who shall supply his place ? 



