2 THE EVOLUTION THEORY 



eortain extent, nioukls it into new forms; or it depends on the 

 continuuUy operatini; forces which make up life, and on the way in 

 wliich these are inlhieneed l>y chan^ini;- external conditions. Which 

 of these alternati\es is correct we can only undertake to determine 

 when we know the phenomena of life, and as far as possible their 

 causes, so that it is indispensable to make ourselves acquainted with 

 these as far as we can. 



When we look at one of the lowest forms of life, such as an 

 Amn'ba or a sincrle-celled Aloa, and reflect that, according to the theory 

 of evolution, the whole realm of creation as we see it now, with Man 

 at its head, has evolved from similar or perhaps even smaller and 

 simpler organisms, it seems at first sight a monstrous assumption, and 

 one which quite contradicts our simplest and most certain observa- 

 tions. For what is more certain than that the animals and plants 

 around us remain the same, as long as we can observe them, not 

 through the lifetime of an individual only, but through centuries, and 

 in the case of many species^ for several thousand years ? 



This being: so, it is intellioible enouoh that the doctrine of 

 evolution, on its first emergence at the end of the eighteenth centurj^, 

 was received with violent opposition, not on the part of the laity only, 

 but by the majority of scientific minds, and instead of being followed 

 up, was at first opposed, then neglected, and finally totally forgotten, 

 to spring up anew in our own day. But even then a host of 

 antagonists ranged themselves against the doctrine, and, not content 

 with loftily ignoring it, made it the subject of the most violent and 

 varied attacks. 



This was the state of affairs when, in 1858, Darwin's book on 

 Tlie Origin of Species appeared, and hoisted the flag of evolution 

 afresh. The struggle that ensued may now he regarded as at an end, 

 at least as far as we are concerned — -that is, in the domain of science. 

 The doctrine of descent has gained the day, and we can confidently 

 say that the Evolution theory has become a permanent possession of 

 science that can never again be taken aw^ay. It forms the foundation 

 of all our theories of the organic world, and all further progress must 

 start from this basis. 



In the course of these lectures, we shall find at every step fresh 

 evidence of the truth of this assertion, wdiich may at first seem all too 

 bold. It is not by any means to be supposed that the whole question 

 in regard to the transformation of organisms and the succession of 

 new forms of life has been answered in full, or that we have now 

 been fortunate enough to solve the riddle of life itself. No ! whether 

 we ever reach that goal or not, we are a long way from it as yet, and 



