9 8 The Scottish Naturalist. 



gathered and registered, science has not discharged her 

 functions with respect to them till it be acknowledged. She 

 has no alternative but to discover, accept, and proclaim what- 

 ever is a law of present experience. Is a polar arrangement 

 in the production of genera part of the evidence of the facts 

 ascertained ? That is the whole question in the first instance. 

 If that were settled affirmatively there would then of course 

 arise a second, as to the cause that might be supposed to deter- 

 mine such a state of things. A question which Forbes did not 

 contemplate, and which would now emerge with an interest 

 and a hopefulness that nothing before the day of Darwin could 

 have given it. 



As to the question, then, of the reality of a polar arrangement 

 of genera in ^geological distribution, what is the evidence at 

 command? It is not entirely to seek. At the very moment 

 when Forbes first broached his theory, there was passing through 

 its second edition the standard work of "Piclet on Palaeozoology." 

 If you had undertaken the simply mechanical task of counting 

 up the genera with the production of which each geological 

 formation was in that work credited, you would have found 

 your figures, somewhat to your surprise in view of this theory, 

 assume an arrangement something like this. (See Table.) 

 This Table, even discounting, as it does, the advantage which 

 the admission of recently originated genera would have given, 

 offers about as polar an arrangement as could well be. There, 

 of a truth, the force that diversifies organic structures to the 

 extent of generic distinctness, as it works along the axis of 

 geological time, begins with maximum intensity of action ; 

 then lowers towards a minimum in Permian and Triassic times ; 

 from that minimum again gathers strength gradually till with a 

 second maximum it ends in the Tertiary, as it had begun with a 

 first in the Silurian, epoch. It is not. known in what detailed 

 and tabulated shape Forbes would have presented the proof 

 which he considered himself to have ready at hand j but this 

 result seems to show that the facts at the time known might have 

 been cast into some shape of preponderating evidence in his 

 favour. It is certainly remarkable. With respect to the 

 Palaeozoic period in Silurian times, the earliest, there are pro- 

 duced more animal forms generically distinct than in all the 

 other later Palaeozoic formations together, and in the Permian 



