INTRODUCTION. Iv 



under a partial retrogression of the causes to which they owed their newly developed 

 organization," 



Very significant is the suggestion which follows, as to the cause of comparatively 

 sudden leaps in the process of evolution : We may suppose some species and indi- 

 viduals to be more able to pass than others, and that many become extinct from ina- 

 bility to accomplish it. Under this point of view, a hiatus, rather than a regular 

 passage, is required between a species and that whence it is supposed to be derived, 

 just as two crystals may occur, nearly identical in composition, but without an insensi- 

 ble gi'adation of intermediate forms ; the laws, both of organic and inorganic matter, 

 requiring something definite, whence the rarity of hybrids and monsters, themselves 

 subject to established laws." He adds, in a foot-note : " The same mineral may crys- 

 talize with three, six, or twelve angles, but not with five or seven. Are the phases of 

 organic morphism subject to less definite laws ? " 



In the year 1850, Professor Joseph Leidy wrote that a slight modification of the 

 essential conditions of life were sufficient to produce the vast variety of living beings 

 upon the globe. 



In 1853 Dr. Jeffries Wyman published a paper on the effect of the absence of light 

 on the development of tadpoles, and in 1867 appeared his Observations and Experi- 

 ments on Living Organisms in Heated Water. Professor Wyman taught the doctrine 

 of evolution as early as 1861, and probably earlier. 



In 1864 B. D. Walsh endeavored to establish the fact that, while the great majority 

 of species may have been formed by natural selection, some originated " by changes in 

 the conditions of life, and especially by change of food." H. J. Clark, in his Mind 

 and Nature (1865), advocated evolution, and even spontaneous generation through 

 physical processes. 



Professor Alpheus Hyatt (in 1866) showed that the development of the individual 

 in the Ammonites agrees with the development of the order to which it belongs, and 

 he afterward showed, by a study of Ammonites of different geological fonnations, that 

 just as there are sudden changes of form in the growth of the individual, so species and 

 genera of one formation replace those of another, in such a manner that one form 

 must have descended from the other, although the differences between the forms are 

 very marked. 



In 1869 Professor E. D. Cope, in his essay on the Origin of Genera, suggested 

 that, by an acceleration or retardation in the development of the animal, generic forms 

 had been produced. He claimed that, " while natural selection operates by the ' preser- 

 vation of the fittest,' retardation and acceleration act without any reference to ' fitness ' 

 at all ; that, instead of being controlled by fitness, it is the controller of fitness." He 

 also remarks that the " transformations of genera may have been rapid and abrupt, 

 and the intervening periods of persistency very long; " in other words (p. 80), genera 

 and higher categories have appeared " in geological history by more or less abrupt 

 transitions, or expression points, rather than by uniformly gradual successions." 



It should be observed, however, that Cope did not enter into the causes which 

 produce acceleration and retardation, but in later papers he has extended and more 

 fully stated his views. 



Marsh's observations, published in 1868, on the transformtion of Siredon into the 

 ordinary gill-less salamander {Ambly stoma), was a step in the same direction, i. e. giving 

 proofs of rapid change in the acquisition of new organs, and modifications of existing 

 ones. 



