468 THE HISTORY OF BIOLOGY 



Species of the same genus are analogous. In connexion with this point he 

 accounts for what he calls the "tendency to reversion" — the frequent and 

 unexpected tendency, especially in domestic animals, for forms to arise hav- 

 ing the characters of the wild species: tame pigeons resembling the wild 

 rock-pigeon, horses with zebra-like streaks, and other similar instances. 



Difficulties of the evolution theory 

 Darwin having thus sought to determine the laws of variation, he takes 

 up for study the difficulties offered by the theory of evolution by means of 

 natural selection. The chapters devoted to this task comprise more than 

 half the book and represent a strange miscellany. As a matter of fact, Darwin 

 acknowledges no limitations to his duty to answer all objections to his the- 

 ory, and he always finds some way out of a difficulty, however desperate it 

 may at first appear. He himself considers that the most difficult phenomenon 

 to explain according to the theory of selection is how the ants' workers 

 have acquired their intelligence; they cannot reproduce themselves and thus 

 transfer their favourable variations by heredity to any offspring. The diffi- 

 culty is solved by the assumption that here it is actually the community as 

 a whole which derives the advantage from variations; the sex-individuals 

 that have produced workers with the best and most advantageous qualities 

 have been victorious in the struggle for existence, and thus have arisen both 

 the highly cultivated worker types and the strongly developed instincts to 

 make slaves, tend aphides, etc. Darwin undertakes another particularly dif- 

 ferent task in seeking to explain how such a complicated organ as the eye 

 came to be formed. This explanation, which was ill received by contempo- 

 rary critics, is certainly rather far-fetched; there is no direct transition be- 

 tween the vertebrate animals' type of eye and that of the Arthropoda — it is 

 not stated why association is not sought with the molluscs instead, in which 

 order the highly developed visual organs of the ink-fish might have served 

 as a transition. — And so the whole work concludes with some general 

 assurances as to the metamorphosing power of selection. It is much easier, 

 of course, to explain the origin of the lungs from the swimming-bladder; 

 on this subject earlier comparative-anatomical observations have been avail- 

 able as a basis of study. Darwin even undertakes to defend the old objection 

 of the sterility of hybrids, which has so often been brought forward in favour 

 of the constancy of species. He differentiates between infertility as the result of 

 crossing species on the one hand, and the sterility of hybrids on the other; 

 as far as the sterility between the species is concerned, he finds that it varies 

 greatly in different organisms — Koelreuter's and Gartner's experiments 

 are especially cited as instances — ■ and the final conclusion is that "accidental 

 and unknown circumstances" are the cause of it in the different cases. The 

 sterility of hybrids, again, is compared with the infertility of wild animals 

 in captivity; each is attributed to the direct influence of external conditions 



