662 , 



are not proportional to each otlier, neither are tliev absolutely equal; 

 in other terms, the smaller animal has, in proportion to its body, a 

 large eye, yet it is absolntely surpassed by that of the larger animal. 

 We find here evidently a similar relation as between the weight of 

 the brain and that of the body, and can try to fix this relation in a 

 similar way, by calculating an exponent of correlation. 



Most fit for this comparative investigation are again species that 

 differ as much as possible in size, and have at the same time 

 absolutely large eyes. Instead of the simple diameter of the eye-ball 

 (which in its shape and in the thickness of the sclerotica is variable) it 

 is preferable to compare the linear sizes of the images on the retina. More 

 than twenty years ago Matthiessen ^) made exact measurements 

 of the sizes of the images on the retina, amongst others in Whales, 

 which together wdth others were already formerly discussed by 

 me. He does not indeed indicate the sizes of the animals them- 

 selves, but if we admit for them the averages of the full grown 

 species, then the error resulting from this insutïïcient information 

 cannot be very great. 



Let us thus compare the largest of the four examined species of 

 Whalebone-Whales, Sibbald's Fin-Whale, with the smallest, the 

 Humpback Whale, and calculate according to what exponent of 

 correlation of the length of the body proportionality with tlie size 

 of the image is obtained ''). 



Proportion of 



linear sizes of the lengths of the body (/) 

 'images (in Millimeters) (in Meters) 



Larger Fin-Whale 



(Balaenoptera Sibbaldi) 



and 



Humpback-Whale 



(Megaptera Boops) 



39.78 

 30.23 



30 

 15 



We find then that on an average the lengths of ihe body must be 

 involved to the power 0.3964 to become pi-oportional to the lengths 



1) L Matthiessen. Die neueren Fortschritte unserer Kentnis von dem optischen 



Baue des Auges der Wirbelthiere. Festschrift fur H. von Helmholtz 1891, p. 62-63. 



^) The Porpoise (mentioned by Matthiessen as "Delpliinus communis") and the 



Whalebone-Whales belong to phylogenetically different orders, Ondontoceles and 



Mysticetes, which differ greatly both in the relative size of the eye and in the 



ephalisalion (this in reverted proportion). Therefore they cannot be compared here. 



