312 GENERAL PHYSIOLOGY 



into chemical relations with the water and its dissolved salts and 

 gases, and thus originated living proteids, i.e., extremely labile 

 compounds, which like other compounds containing the cyanogen- 

 radical are distinguished by their tendency toward decomposition and 

 polymerisation, and which form the essential constituents of living 

 substance. This first living substance, which was formed spon- 

 taneously out of lifeless substance, was very simple and showed 

 no differentiations. It is very probable that it did not have the 

 morphological value of cells, i.e., that its mass was not yet 

 separated into different substances, such as nucleus and proto- 

 plasm, but rather was .homogeneous in all its parts, as Haeckel 

 assumes for his Monera. 



Such an idea of the origin of living substance has at present 

 some degree of probability in its favour. It is quite possible that 

 in the future it will be considerably modified in its details. Yet 

 further speculation at present regarding the details is of little 

 value, since the stage upon which living substance made its first 

 appearance and the conditions then prevailing are known so in- 

 definitely. But with living substance already present upon the 

 earth we are upon firmer ground ; for here is the point where the 

 doctrine of descent, founded by Lamarck and Darwin, and de- 

 veloped especially by Haeckel, Weismann and their pupils, comes 

 in and elucidates the farther history of this substance down to the 

 present day. 



It would lie outside the purpose of these pages to speak of the 

 whole enormous complex of ideas that led to the founding of the 

 doctrine of descent. It is sufficient to point to the chief factors, 

 the correctness of which no thinking man of science at present 

 doubts. 



As is well known, the theory of descent teaches that all the 

 multifarious organisms that live to-day and have lived at any 

 time upon the earth's surface are derived in unbroken descent 

 from the first and simplest living substance that originated from 

 lifeless substances, and that, therefore, all organisms stand in true 

 genetic relationship to one another. The continuity of the organic 

 series during historic time needs no special proof; for simple 

 observation shows that every organism is derived from another 

 organism similar to it, that the continuity of descent is never 

 broken. But for the long geological periods elapsing between the 

 appearance of the first organisms and historic time, direct observa- 

 tion is naturally wanting. Here nature has preserved certain 

 records in which are found entered, although more or less incom- 

 pletely, the history of the evolution of the organic race. 



The first record is deciphered by Palaeontology, or the science of 

 fossils. Fossils are the testimony that nature has laid down 

 in the strata of the earth's crust regarding the existence and 

 character of earlier organisms. By the study of fossils palseon- 



