CXxii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1909, 



elsewhere, and the crania in our caves are the relics of succes- 

 sive waves of migratory invasion. The races they represent are 

 probably the final terms of distinct evolutional series : they were 

 comparatively stable products, and their descendants, now repre- 

 sented by the Australians, Bushmen, and Eskimos, retain to a large 

 extent the cranial characteristics, and especially the cranial capacity, 

 which they possessed when they occupied a large part of the 

 continent of Europe. Whatever other changes the human brain 

 may have undergone since the close of Paheolithic times, it has 

 certainly not increased in volume. 



A consideration of the rapid transformation of the Equidae and 

 Man, in such striking contrast to the almost complete fixity of 

 type in Lingida and Discina, suggests a question as to the origin of 

 the difference. What is it that induces in some of the descendants 

 of a particular form a stable and in others a labile state ? If 

 changes in the environment afford the necessary stimulus, how is it 

 that similar organisms respond so differently to this stimulus ? 

 The answer to this question no doubt depends on the value of the 

 qualification ' similar ' ; what Leibniz assumed with regard to his 

 individuals is true also for organisms — no two are absolutely alike, 

 and our difficulties often arise from the use of the same word to 

 include different things : but this is only a restatement of the fact 

 of variation. 



It will be observed that in both the instances we have just con- 

 sidered, the known facts provide ample time for a chronologically 

 slow and continuous evolution of what appear to be rapidly 

 changing organisms ; but it is doubtful whether this conclusion 

 could be still maintained if we assumed that the variations they 

 sum up were absolutely indeterminate ; what seems rather to be 

 suggested is a succession of similar variations added one to another 

 in a linear series. If this should prove to be the case, the next 

 word on this subject might be expected neither from the zoologist 

 nor the geologist, but from the student of what is known as 

 Psychophysics. Samuel Butler's suggestive book on ' Life and 

 Habit ' may possibly prove an adumbration of a new and wide- 

 reaching explanation of the origin of species, embracing all that 

 is true in the great work of Darwin and of his brilliant precursor 

 Lamarck. 



