302 SUMMARY OF CURRENT RESEARCHES RELATING TO 



compares with it in importance. The direct influence of the change of 

 temperature is not very great, and in this respect the difference of 

 temperature in the lake corresponds to temperature-differences in general. 

 Most plants and animals of temperate regions are not particularly sensi- 

 tive to a change of a few degrees of temperature. For some species, 

 however, the change from warm to cool water constitutes the factor 

 which determines their vertical distribution. Indirectly, the effect of 

 the thermocline is far greater. The stagnation of the lower water, with 

 its attendant chemical results, causes a sharp limitation of the distribu- 

 tion of the animal life in many lakes. The thermocline in these lakes 

 marks the limit of the thriving of algae and thus directly limits the 

 distribution of plants and indirectly that of the animals which feed upon 

 them. In all lakes the thermocline has an evident influence upon 

 distribution, and although it is by no means an impassable barrier, most 

 species of plankton animals live, by preference, either above or below it. 



Origin of Markings of Organisms.* — A. S. Packard discusses his 

 theory that the markings of organisms are due to the physical rather 

 than the biological environment. The alleged cases of Mullerian 

 mimicry can be explained by convergence due to such causes. He 

 regards the attacks of birds upon insects as a negligible factor. Resem- 

 blances in coloration and markings are the result of pigmentation 

 caused by exposure to the combined effects of sunlight and shade. 

 They are due to the repetition of the fundamental colours, brown, black, 

 red, yellow, in insects of different orders, as well as animals of different 

 classes, living exposed to direct sunlight, and often having exceptional 

 diurnal or light-loving habits in contrast to the lucifugous habits of the 

 other species of the genus, family or order. The similarity of design 

 appears in many, if not most, cases to be due to the repetition of mark- 

 ings with identical shapes or patterns, i.e. lines, bars, which are eventually 

 broken up into spots and repeated ad infinitum, owing to the economy 

 of material and design, differing in detail in different groups owing to 

 their different origin and hereditary constitution. Such markings 

 probably gradually arose in a given region simultaneously in all 

 the individuals, and not as a variation in a single individual, which is 

 supposed to have been favoured in the struggle for existence. While 

 the initial causes, therefore, are Lamarckian, natural selection as a 

 preservative process may form a subordinate factor. It is pointed out 

 that stripes, bars, and spots occurred on the wings of Palaeozoic insects 

 which flourished before the appearance of birds and even of modern 

 types of lizards. 



Pre-Aristotelian Zoology.f — Rudolph Burckhardt gives an analysis 

 of the zoological or dietetic part of the Corpus Hippocraticum and 

 compares what he calls " das koische Tiersystem " and " die knidische 

 Tierfolge " with Aristotle's classification. After careful consideration he 

 comes to the conclusion that although there were pre- Aristotelian hints 

 at orderly arrangement, Aristotle was the first to consciously employ the 

 principles of zoological taxonomy. 



* Proc. Amer. Phil. Soc. xliii. (1904) pp. 393-450. 

 t Verh. Nat. Ges. Basel, xv. (1904) pp. 377-414. 



