INTRODUCTION. lyii 



have remodelled the whole psychology, as it were, of the animals of our country ? 

 Instincts vary with the varying structure and form of the animals. Change the sur- 

 roundings, and at once the mode of life and psychology of the organism begin to 

 undergo a revolution. These changes may result in the gradual extinction of whole 

 assemblages of animals, which are as gradually replaced by new faunas." 



Mr. J. A. Allen has (in his works on the variation of birds, published in 1871, and 

 especially in subsequent papers) shown the influence of climate and temperature in 

 directly inducing specific ghanges, without the agency of natural selection. 



In the American Naturalist for March, 1877, Mr. W. H. Dall published a thought- 

 ful article On a Provisional Hypothesis of Saltatory Evolution. He realizes that 

 " leaps, gaps, saltations, or whatever they may be called, do occur " in the evolution of 

 forms. Mr. Dall remarks that " the apparent leaps which Nature occasionally exhibits 

 may still be perfectly in accordance with the view that all change is by minute differ- 

 ences, gradually accumulated, in response to the environment." 



The articles of Mr. W. H. Edwards on dimorphism and seasonal variation in our 

 butterflies (Canadian Entomologist, 1877) throw light on the production of species by 

 climatic changes, and, with Weismann's work on this subject, published in Germany, 

 clearly show how many species were called into being by the geological and especially 

 climatic changes wrought by the advent and departure of the glacial period. All 

 these works show how many are the causes, much more fundamental than natural 

 selection, which have played their part in the origin of the varieties, which have been, 

 however, preserved by natural selection. 



In his work on sjDonges (published May, 1877) Professor Hyatt gives a large 

 number of novel facts, showing how greatly sponges are modified in form by the nature ' 

 of the sea-bottom and the temperature of the water. The same line of thought is 

 extended in his elaborate treatise on the Steinheim shells, published in 1882. 



It should also be said that Huxley has incidentally observed : " We greatly suspect 

 that Nature does make considerable jumps in the way of variation now and then, and 

 that these saltations give rise to some of the gaps which appear to exist in the series 

 of known forms." Galton, Mivart, and W. K. Brooks have also favored the view that 

 saltations may occur. 



Two recent addresses, one by Professor Le Conte of California, and the other by 

 Mr. Clarence King (1877), have forcibly set forth the results upon organic life of the 

 revolutions in the history of the earth. Professor Le Conte, speaking as a geologist, 

 represents " the organic kingdom as lying, as it were, passive and plastic under the 

 moulding hands of the environment." He speaks of "general evolution, changes 

 of organisms, whether slow or rapid, as produced by varying pressure of external 

 conditions." Again, he ably remarks : " There seems good reason to believe that the 

 evolution of the organic kingdom, like the evolution of society, and even of the 

 individual, has its periods of rapid movement and its periods of comparative repose 

 and readjustment of equilibrium." He illustrates this by .referring to the change from 

 the cretaceous to the tertiary period, involving not only a change in climate, but of 

 salt water to fresh, and the extinction of some marine animals, as well as the transmu- 

 tation of others into fresh-water species. Le Conte gives the first place to pressure on 

 the organism resulting from changed physical conditions, and the second place to 

 natural selection. 



Mr. Clarence King, with his experience as a geologist in the west, has advocated 

 catastrophism in geology, and shows the inadequacy of uniformitarianism in entii-ely 



