244 GROWTH PRINCIPLES AND THEORY 2 



(Rose, 1956), ofurodeles (Homeyer, 1951 ; Nolte, 1953), of insects (Goossen, 1949) ; 

 liver and other organs (Rensch, 1948b); pancreas (Padour, 1950); eyes of 

 urodeles (Moller, 1950); various organs of insects (Partmann, 1948). Pictures 

 of the mesogastric gland of insects of different body size are reproduced as an 

 example (Fig. 44). 



For example, the eyes of larger vertebrates have more retina cells than those of related 

 smaller species (Moller, 1950); similarly, larger insect species have more ommatidia 

 (Partmann, 1948); increase in the number of photoreceptors makes possible a finer 

 resolution of the visual image. If the brain increases in absolute size, it contains, in insects, 

 a greater number of ganglion cells, and in mammals, larger ganglion cells. In both cases, 

 more dendrites are provided, and consequently a number of synaptic junctions and 



Fig. 45. Pyramid cells from homologous parts of the cortex of two rodents of different 

 size. Left, water hog {Hydrochoerus capybara), right, guinea pig {Cavia cobaya). Same magni- 

 fication. After Spina Franca Netto from Rensch, 1954. 



more complicated functions (Fig. 45). Furthermore, it is particularly the neencephalic 

 parts of the brain and among these, the most complicated regions which increase in relative 

 size with evolutionary increase of body size. Hence increase of body size makes possible 

 more elaborate behavior patterns, both innate and learned. Larger races of domestic fowl 

 are able to learn a greater number of visual problems than smaller ones (Altevogt, 1951) ; 

 although white mice learn faster than white rats, the latter remember longer (Boxberger, 

 1953) ; memory in the larger Xiphophorus is better than in the small Lebistes (Rensch, 1954), 

 etc. The elephant as a mammal with especially large absolute brain size, "understands" 

 up to 20-24 verbal commands, and is able to discriminate between some 20 pairs of visual 

 stimuli; it also shows a well-developed capacity to abstract by distinguishing, e.g., numbers 

 of dots irrespective of their position, variations of a cross pattern against a circle, etc. 

 (Rensch, 1956, 1957). In summary it can be stated that there are differences in learning 

 capacity between related races or species correlated with body size. The speed of learning 

 possibly depends on the intensity of metabolism (small forms learn faster), while larger 

 forms have a longer memory (Rensch, 1954, 1958). 



The physiological consequences of changes in body size are apparent from the facts 

 discussed previously: Reduction of weight-specific metabolic rate with increasing body size 

 (in mammals following the 3/4-power rule) ; reduction of pulse and respiratory frequency; 

 lower blood sugar level in connection with the reduction of metabolic rate; prolonged 

 development, etc. (Rensch, 1954). An example of the evolutionary efiect of these physio- 

 logical factors is provided by the giant forms appearing in glacial periods (elephants, 

 Megaceros, large felines) ; decrease of temperature favored increase in size in the sense of 

 Bergmann's rule. 



