664 LECTURE XXIX. 



witness of long-forgotten ages ! We also find many insects which are sharply 

 confined to the Alps and to the Far North. Many a paleontologic discovery 

 serves to form a bridge between two apparently foreign fields, and at one 

 stroke changes assumptions to indisputable proofs. The bottoms of our 

 Alpine lakes are covered with forms of life which we encounter again only 

 in the arctic regions. 1 How interesting it would be to turn our physiologi- 

 cal chemistry into similar channels! We have hardly begun to advance 

 in this direction, for our present methods are still unable to follow the flight 

 of thought, and our understanding of the chemical processes taking place 

 in the different organisms is still too limited for us to make comparative 

 studies. Nevertheless this goal should be regarded as most worthy of 

 attaining. To be sure, there are a great many isolated facts and a multi- 

 tude of observations concerning the organisms of different kinds of plants 

 and animals, but they are far from being of equal value, and it remains to 

 unite our knowledge of certain processes into a continuous chain. We 

 can, however, call attention to certain facts which indicate that not only 

 every species of animal but even each individual is to be regarded as one 

 which is characteristic and limited in its general metabolism. 2 



Let us consider for the moment the great multiplicity of forms in the 

 animal kingdom. In what a contrast the tissues of these morphologically 

 so different beings must stand! Consider the vertebrates. Everywhere 

 we find the same physiological function, the same tissue, the same organ. 

 Not only is this true of the external appea'rance, but even the finer struc- 

 ture shows a great similarity. In spite of this fact, the same organs of 

 different species of animals are very different in their metabolism, and 

 again the chemical composition of these organs and tissues must be char- 

 acteristic for each species of animals and perhaps for every individual. 

 Herein lies the reason for the differences in their metabolism. Let us see 

 what right we have to compare the purely morphological differentiation 

 of the various kinds of life into classes, families, and species with 

 physiological-chemical limitations, especially as regards the species. If 

 we consider physiological-chemical investigation as a whole, we shall find 

 that it extends in two directions. On the one hand many apparently 

 dissimilar elements are united by a common band to form a large whole, 

 and on the other hand many functions which were apparently identical 

 have, after careful study of the individual processes, been found to be 

 different in nature, and thus limitations have been found to exist where 

 none were suspected. Thus it was formerly believed that there was a 



i Cf. F. Zschokke: Die Tierwelt der Schweiz in ihren Beziehung zur Eiszeit, Basel, 

 1901. 



3 Cf. Huppert: Ueber die Erhaltung der Arteigenschaften, Prag, 1896. Franz Ham- 

 burger: Arteigenheit und Assimilation, Leipsic and Vienna, 1903. Emil Abderhalden: 

 Naturwissenschaftliche Rundschau, 19, No. 44 (1904). 



