142 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



This almost total blindness in the mole is the result solely of complete degeneration of the optic nerve, so that 

 the images which are probably formed in the eye itself can never be transmitted to the animal's consciousness. Oc- 

 casionally, however, the mole even can see a little, for it has been found that both optic nerves are not always 

 degenerate in the same individual, so that one eye may remain in communication with the brain while the other has 

 no connection with it. In the embryo of the mole, however, and without exception, both eyes are originally con- 

 nected with the brain by well-developed optic nerves, and so theoretically efficient. This may indeed be regarded 

 as a perfectly conclusive proof that the blind mole is descended from progenitors that could see ; it would seem, too, 

 to prove that the blindness of the fully grown animal is the result not of inheritance, but of the directly injurious 

 effects of darkness on the optic nerve in each individual.* 



It may be objected, however, that each mole certainly inherits a tendency to weakness and 

 atrophy of the optic nerves, just as the children of consumptive or strumous parents inherit a 

 tendency to those diseases, and that when the conditions are favorable the disease manifests itself. 

 We know there have been many generations of blind or partially blind moles, and it would be 

 strange if heredity did not at a certain age act in such a case, and would not for at least a few 

 generations even if the moles were kept out of the darkness. We have in the atrophy of the optic 

 nerves of the mole a parallel case in the blind Myriopod Pseudotremia cavemarum, where the eyes 

 survive, but the optic nerve is wanting, as also in a less marked degree in Ccecidotcea stygia. 



THE FACTORS OF ORGANIC EVOLUTION. 



The study of the conditions of existence in caves is of special value, because such conditions 

 are so unusual and abnormal and the results upon certain organs so easily appreciated. It is by 

 a study of life under unusual conditions that the attention is aroused and interest excited, and 

 after acquiring experience in dealing with the more palpable, because somewhat abnormal, circum- 

 stances under which organisms exist, we can then more easily observe the effects of changes of 

 ordinary conditions upon the organism. 



From a study of cave life, of organisms existing in saline and in heated waters, of plants and 

 animals exposed to great cold in alpine or polar regions, of those living in hot, dry deserts, we 

 oan turn to an examination of the results of adaptation to a parasitic mode of life. The strange 

 modification of form, owing to disuse, in internal as well as external parasites of different orders 

 mid classes, the change of host necessitated, and the intensity of the struggle for existence in 

 animals living under such exceptional conditions, embryology proving that they have arisen from 

 animals of normal organization ; such studies as these are of fundamental importance in a dis- 

 cussion of the origin of species and higher categories. Moreover, the study of the results of the 

 incoming and cessation of the Glacial epoch, the effects on life arising from the elevation and 

 depression of the land, involving not only change of land surfaces, but a change of climate; it 

 is by a study of such marked changes as these in the conditions of life that we are prepared to 

 examine the more subtle causes of variation throughout the organic world in general. 



After the foregoing pages were written we read with much interest Mr. Herbert Spencer's 

 recent essays entitled " The Factors of Organic Evolution." t While that author, it appears to us, 

 lays too great stress on Dr. Erasmus Darwin's views, as compared with Lamarck's; the author of 

 the Philosophie Zoologique having been a professional botanist and zoologist as well as a naturalist 

 of the first rank, it is noteworthy that he sees clearly that natural selection is not the sole factor in 

 organic evolution, as will be seen by the general drift of his essays, by his quoting with approval 

 Huxley's significant remark that " Science commits suicide when it adopts a creed," and by the 

 following extracts from his own essays : 



But now, recognizing in full this process brought into clear view by Mr, Darwin, and traced out by him with so 

 much care and skill, can we conclude that, taken alone, it accounts for organic evolution ? Has the natural selection 

 of favorable variations been the sole factor ? On critically examining the evidence we shall find reason to think that 

 it by no means explains all that has to be explained (p. 9). 



During that earlier period, when he was discovering the multitudinous cases in which his own 

 hypothesis afforded solutions, and simultaneously believing how utterly futile in these multitudi- 

 nous cases was the hypothesis propounded by his grandfather and Lamarck, Mr. Darwin was, not 



* Animal Life, etc., pp. 79, 80. 



tNew York, 1887, reprinted from The Nineteenth Century for April and May, 1886. 



