248 



of a species, there is no nerve cell division; tiiese cells only increase 

 in volume, wliicli tliey also do with the origination of larger 

 species. For this increase of the nerve cell volume is a mechanical 

 necessity, as may appear further below. 



That phylogenetic increase of the volume of the brain is actually 

 brought al)oul by cell division, associated with equivalent increase 

 of the separate cell volume, is also proved by the fact that in 

 related, l)iit heteroneuric species, with equal body weight, the volumes 

 (or weights) of the brain or — what comes to the same — with 

 inequal body weight, the calculated coetTicients of cephalisation, 

 in many cases, are to each other as J, 2,3,4. The cranial capacities 

 of the Oliimpanzeo (450 c.c), of Pithecanthropus (900 c.c), and 

 of the male Australian aboriginal (1350 c.c.) are to each other as 

 the numbers 1:2:3. The coefficient of cephalisation of the Man-like 

 Apes is twice that of the Old World Monkeys and Baboons; Cebus 

 has double the cephalisation coefficient of Chrysothrix; in the 

 Megachiroptera it is twice Ihat of the Microchiroptera. The coefficient 

 of the Tiee Shrew (Tupaja) is four times that of the Common Shrew 

 (Sorex) and the Musk Shrew (Crocidura). The coefficients of the 

 genera Mus, Lepus, and Sciurus are to each other as 1:2:3. The 

 genera Tapir, Sus, and Hippopotamus have a coefficient of cepha- 

 lisation half as great as that of the Horses, the Deer, the Giraffe, 

 the Antilopes, and tlie Oxen. The Chevi-otain (Tragulus) also has 

 a coefficient only half so great as the modern-type Ruminants. It 

 is extremely interesting that among the Mustelidae, the Polecat 

 (Putorius putorius), the Stoat (Putorius ermineus), and tlie Weasel 

 (Putorius nivalis) possess a coefficient of cephalisation only half so 

 great as the Beech-Marten (Mustela foina) and the Pine-Marten (Mustela 

 martes). In this respect the Badger (Meles) agrees with the former, 

 the Otter (Lutra) with the latter group. 



We meet here with an important phenomenon, analogous to the 

 "parameter-law" of crystals, and, undoubtedly, intimately connected 

 with the polyploidy of nuclei and consequent rational increase of 

 cell volume. 



It may, further, be pointed out that most of the heteroneuric 

 species mentioned with low cephalisation, are small, in comparison 

 with the allied species with high cephalisation. This proves that the 

 phylogenetic growth of the brain, in which — different from what 



Monkeys are to each other as 8 : 24 : 70, the absolute number of cells increases 

 considerably more than would correspond with the same size of body of homo- 

 neuric species. In the nearly homoneuric Gibbon (Siamang) and Chimpanzee those 

 numbers are 3160 and 1765, and the brain weights to each other as about 1 : 3. 



