200 SUMMAEY OF THE [Chap. XHL 



be closely related to each other ; and should likewise 

 be related, but less closely, to those of the nearest 

 continent, or other source whence immigrants might 

 have been derived. We can see why, if there exist 

 very closely allied or representative species in two 

 areas, however distani from each other, some identical 

 species will almost always there be found. 



As the late Edward Forbes often insisted, there is a 

 striking j)arallelism in the laws of life throughout time 

 and space ; the laws governing the succession of forms 

 in past times being nearly the same with those govern- 

 ing at the present time the differences in different 

 areas. We see this in many facts. The endurance of 

 each species and group of species is continuous in time ; 

 for tlie apparent exceptions to the rule are so few, that 

 they may fairly be attributed to our not ha\"ing as yet 

 discovered in an intermediate deposit certain forms 

 which are absent in it, but which occur both above and 

 below : so in space, it certainly is the general rule that 

 the area inhabited by a single species, or by a group of 

 species, is continuous, and the exceptions, which are not 

 rare, may, as I have attempted to show, be accounted for 

 by former migrations under different circumstances, or 

 through occasional means of transport, or by the species 

 having become extinct in the intermediate tracts. 

 Both in time and space species and groups of species 

 have their points of maximum development. Groups 

 of species, living during the same period of time, or 

 living within the same area, are often characterised by 

 trifling features in common, as of sculpture or colour- 

 In looking to the long succession of past ages, as in 

 looking to distant provinces throughout the world, we 

 tind that species in certain classes differ little from each 



