cxxxvi GENERAL SUMMARY OF SCIENTIFIC AND 



clearly changed in the old males ; and in the sacs in which 

 the outer sexual organs lie we find a space which is very 

 similar to the ovisac of the Branchipus ferox Gr. In such 

 old males the spermatic particles are very clearly enlarged. 

 How far these changes could go on he could not say, since 

 this Branchipus is short-lived. These changes in the sexual 

 organs are especially marked in old individuals; and he fur- 

 ther remarks that such misshapen forms often occurred in 

 the salt-pools after heavy rains" (Siebold & Kolliker's Zeit- 

 schrift, 1872, p. 293). 



Such facts as these show how desirable collections in very 

 large numbers, at different seasons of the year and from dif- 

 ferent localities, are for the proper study of these animals. 

 Moreover, they are among the most important facts showing 

 how new generic and specific forms, as well as a sexual re- 

 production, arise in consequence of changes in the physical 

 surroundings of animals. 



One of the most important contributions of the year to the 

 theory of evolution are the results of Professor Hyatt's stud- 

 ies on the Ammonites. He finds every where throughout the 

 group two methods of development one by a slow accumu- 

 lation of differences, according to the Darwinian theory ; the 

 other by their quick or sudden production, according to the 

 law of acceleration, as explained by Cope and himself. 



Pouchet has shown that in fishes which are known to 

 change their color bv living on different bottoms, li<jht and 

 sandy, or dark and muddy, the change is produced through 

 the nervous system. In fishes whose eyes were put out such 

 changes were not brought about. It seems evident, then, that 

 the change of color is caused by the action of light on the 

 retina of the eye. From this fact of a change in color by the 

 presence or absence of the eyes, Dr. Packard has suggested 

 that in the blind fish of the Mammoth Cave we can account 

 for the absence of color in the skin, and its white appear- 

 ance. Having no eyes, no stimulus is conveyed to the pig- 

 ment cells of the skin ; they are aborted, and consequently 

 the skin turns white. The other blind Crustacea and insects 

 are probably rendered white from the same cause. 



In the geographical distribution of animals, it has been 

 found that the Bepidoptera of North America probably fol- 

 low, as regards size, a law the reverse of that established by 



