5o8 



NATURE 



[April 3, 1890 



scanty, and " lack of food as well as the absence of light 

 was one of the factors concerned in the diminution of 

 size and in the slenderness of blind cave animals as com- 

 pared with their lucicolous allies." 



The effect of total darkness upon animals is twofold. 

 Firstly, colour is either entirely or partially bleached, 

 and, secondly, the sense of sight is lost. Eyesight may 

 be lost in various ways. Either the optic lobes and 

 nerves may atrophy, while the retina, pigment, and lens 

 remain more or less persistent ; or the optic lobes and 

 nerves may persist, while the retina and eye-facets 

 atrophy ; or, again, the whole of the optic apparatus may 

 atrophy. Examples of all these cases are given in the 

 important chapter which is devoted to a description of 

 the anatomy of the brain and eyes of certain blind 

 Arthropods, and illustrated by numerous drawings of 

 sections through various regions of the head. 



It is argued that this atrophy must be comparatively 

 sudden and wholesale, because no series of individuals 

 has been found with the optic lobes or nerves in different 

 stages of disappearance. Transitional forms have been 

 observed with eyes with a varying number of crystalline 

 lenses, as in the case of Chthoniiis ; those individuals 

 which live near the mouth of the cave have better deve- 

 loped eyes than those which live far in. And surely, 

 on further examination, more transitional forms will be 

 discovered, as animals must be continually getting into 

 the caves from the outside ; their descendants becoming 

 gradually adapted for cave life, until they finally reach the 

 degree of modification of the present older occupants. 



As the sense of sight diminishes, it is compensated by 

 an increase of the delicacy of other senses. The tactile 

 and olfactory senses are rendered more sensitive, the 

 appendages become much more slender, and the blind 

 form is altogether more timid and cautious than its eyed 

 allies, as has been particularly noticed in the blind cray- 

 fish. 



The last part of this memoir deals with what is of 

 most general interest to the biologist, viz. the bearing of 

 these facts upon the theories of evolution. The author 

 states that here the term " natural selection " expresses 

 the result of a series of causes rather than any one cause 

 in itself. The most important of these causes are : the 

 change of etivironment, from light to partial or total dark- 

 ness, involving diminution of food, the disuse and loss of 

 certain organs, with compensation as has been mentioned 

 above ; adaptation, enabling the more plastic forms to 

 survive and perpetuate the stock ; heredity ,\i\\\<^ operates 

 to secure the future permanence of the newly originated 

 forms— the longer it acts, the earlier will the inherited 

 characters appear in the development of the animal ; 

 and, lastly, isolation, which, after adaptation and heredity 

 have established the typical characters, prevents inter- 

 crossing with out-door forms, and thus insures the 

 permanence of these characters. 



The author adduces facts which seem to prove that the 

 organic adaptations to a life in darkness may have been 

 induced after but a few generations, perhaps one or two 

 only, resulting in the comparatively rapid evolution of 

 cave species. -If that be the case, then, there is no 

 reason why they should not be produced artificially, but 

 at present no experiments have been made to prove the 

 mutual convertibility of cave species and their lucicolous 



allies. If a cave species could be made to revert to an 

 epigean form by keeping it for a number of generations 

 in a gradually increasing amount of light ; and if, on the 

 other hand, a lucicolous species could be changed into a 

 cave form by a converse process, the theory of occasional 

 rapid evolution due to sudden changes in the environ- 

 ment would receive its final proof. 



Mr. Packard draws attention to the interesting parallel 

 between the life of the abysses of oceans and lakes and 

 that of caves. In both cases vegetable life is almost 

 absent, and a large proportion of the animal forms have 

 become similarly modified with regard to the degeneration 

 of the optic organs and corresponding development of 

 other organs as compensation. But while caves have 

 only been populated comparatively recently, the ocean 

 abysses have had inhabitants for a very much longer time, 

 and consequently these have had time to become much 

 more highly specialized than the inhabitants of caves. 



This most valuable , contribution terminates with a 

 bibliography containing the titles of previous publications 

 on the subject, and we must not omit to mention that in 

 a separate chapter a list is given of the known non- 

 cavernicolous blind animals. As far as the higher classes 

 are concerned, this list contains about the same number 

 of species as the one of the blind cave-dwelling forms. 



R. T. G. 



LINEAR DIFFERENTIAL EQUATIONS. 

 A Treatise on Linear Differential Eqtcations. By 

 Thomas Craig, Ph.D. Vol. I. Equations with Uni- 

 form Coefficients. (New York : John Wiley and 

 Sons, 1889.) 



TREATISES on this subject have been somewhat 

 numerous of late. We recently noticed in these 

 columns an excellent, but fairly elementary work, " On 

 Ordinary and Partial Differential Equations," by Prof. 

 Woolsey Johnson. The student who wishes to enter on 

 the profitable perusal of the book before us must be well 

 versed in all the ordinary modes of procedure,* and then 

 he will find that Dr. Craig is well qualified to lead him 

 through the intricate windings of this difficult branch of 

 mathematics. The advanced student will find the author's 

 analysesof usetohimwhilstreadingthevariousoriginal me- 

 moirs here introduced to him, for the first time, in English. 

 Some may remember that Mr. Forsyth, in his classical 

 treatise, omitted the investigations of Fuchs, the recent 

 researches of Hermite and Halphen. contented himselt 

 with a slight sketch of Jacobi's method for partial differ- 

 ential equations, and did not at all touch upon the 

 methods of Cauchy, Lie, and Mayer. The consideration 

 of these matters he reserved for a future volume. 



The theory of the subject before us, i.e. of linear differ- 

 ential equations, almost owes its origin, in Dr. Craig's 

 opinion, to two memoirs by Fuchs, published in vols. 

 Ixvi. and Ixviii. of Crelle's Journal {id,66, 1868) : — 



"Previous to this the only class of linear differential 

 equations for which a general method of integration was 

 known, was the class of equations with constant coeffi- 

 cients, including, of course, Legendre's well-known equa- 

 tion, which is immediately transformable into one with 



' " The reader is of course supposed to be familif r with the ordinary 

 ele nentary the jry of diflf ;rential equations " (p. 32). ^^_^ _ 



