OUTLOOK. 665 



great gulf between the animal and vegetable worlds. There was not 

 supposed to be anything in common between them as regards their 

 chemical processes and their metabolism. The plant cells were alone 

 assumed to build up organic substances, or, in other words, effect syntheses, 

 whereas the animal cells were assumed to break down only. Wohler's 

 discovery that benzoic acid is changed to hippuric acid in the animal 

 organism made the first breach in the wall separating the two kingdoms. 

 Then in rapid succession bridge after bridge has been built between these 

 apparently so distinct fields, so that to-day a common band unites the 

 animal and vegetable worlds. With this knowledge as a basis, it was 

 then desirable -to study more closely the differences between these two 

 kingdoms, and also to unite them more closely by numerous intermediate 

 stages. 



A few examples may illustrate the significance which, from a physio- 

 logical-chemical standpoint, governs the conception of the species. 



The characteristic mark of distinction of mammals, the mammary 

 glands, deliver a secretion, the milk, which is quite uniform in nature. 

 The milk from an animal almost always has a very similar qualitative 

 composition, although it varies quantitatively somewhat. Each species, 

 however, has its own characteristic milk, and this is true not only of the 

 mineral constituents, but of the organic matter contained in it as well. 1 

 In fact, we have reason to believe that even qualitatively the different kinds 

 of milk differ from one another. Although our present knowledge does 

 not suffice to characterize these differences more precisely, and indeed 

 the different kinds of casein appear to us as identical, we must not forget 

 that we are never justified in deciding from the qualitative and quantita- 

 tive composition of the cleavage-products whether the original proteins 

 under investigation are identical. In the arrangement of these constit- 

 uents in the original molecule, to say nothing of the other kinds of isomer- 

 ism, there are countless possibilities. 



Furthermore, let us consider the blood of different animals. In every 

 case it has the same function, the same physiological significance, and mor- 

 phologically the most far-reaching similarity. We always find blood- 

 corpuscles and plasma. What a remarkable similarity there is between 

 human blood and sheep's blood, and yet the sad experiences which have 

 resulted from the attempts at substituting the latter for the former have 

 proved that deep-seated differences must exist. The blood-corpuscles of 

 mammals all contain hemoglobin as a characteristic constituent. Its 

 function is invariably the same, and yet the hemoglobin is specific for 

 each different species, as is apparent from the external relations alone, 

 such as the crystalline form and solubility. Thus the hemoglobin of the 



i Emil Abderhalden: Z. physiol. Chem. 26, 487 (1899); 27, 408, 594 (1899). Cf. 

 Lecture XVII, p. 404. 



