INTRODUCTION. liii 



Weismaiin, in las suggestive work, Studit's in i\w. Thuoiy of Descent, (1875-70), 

 concludes from his extended investigations on seasonal dimorijhisni, " that differences 

 of specific value can originate througli the direct action of external conditions of life 

 only. ... A species is only caused to change through the influence of changing 

 external conditions of life, this change being in a fixed direction which entirely de- 

 pends on the physical nature of the varying organism, and is different in different 

 species or even in the two sexes of the same species." 



Weismann has certainly proved that new species arise by difi'erences in climate, 

 while he also (in a note to the English edition) concedes that sexual selection plays a 

 very important jjart in the markings and coloring of butterflies, but he significantly 

 adds, " that a change produced directly by climate may be still further increased l)y 

 sexual selection." 



A second point, and one of particular interest, which the author claims to be eluci- 

 dated by seasonal dimorphism, is " the origin of variability." Having shown that 

 "secondary forms are for the most part considerably more variable than jjrimary 

 forms," it follows that " similar external influences either induce different changes in 

 the different individuals of a species, or else change all individuals in the same manner, 

 variability arising only from the unequal time in which the individuals are exposed to 

 the external influence. The latter is undoubtedly the case, as appears from the differ- 

 ences which are shown by the various individuals of a secondary form. These are," 

 he adds, giving his proofs, "always only differences of degree and not of kind." He 

 shows that allied species and genera, and even entire families (Pieridse), "are 

 changed by similar external inducing causes in the same manner, or better, in the 

 same direction." 



In his Ursprung und der Princip des Functionswechsel, (1875), Dr. A. Dohrn 

 states his belief that new habits induce the organs to exercise ajiparently new func- 

 tions, which were latent or only partly developed under the original conditions of the 

 surroundings. 



Another work, laden with facts, with not much space wasted on theories, is Semper's 

 Animal Life as affected by the Xatural Conditions of Existence (1877-81). This is 

 the first general work especially devoted to an attempt to discover the causes of 

 variation in animals. As the author says in his preface, "It appears to me that of all 

 the properties of the animal organism, variability is that which may first and most 

 easily be traced by exact investigation to its eflicient causes ; and, as it is beyond a 

 doubt the subject around which at the present moment the strife of opinions is most 

 violent, it is that which will repay the trouble of closer research." An enumeration of 

 the subjects treated in the respective cha])ters of this work will give one an idea of the 

 way in which this diflicult subject should be studied : food and its influence; the influ- 

 ence of light, of temperature, of stagnant water, of a still atmosphere, of water in 

 motion; curients as a means of extending or hindering the distribution of species, and 

 the influence of living organisms on animals. 



In the United States a number of naturalists have advocated what may be called 

 neo-Lamarckian views of evolution, especially the concejttion that in some cases rapid 

 evolution may occur. The present writer, contrary to pure Darwinians, believes that 

 many species, but more especially types of genera and families, have been produced by 

 changes in the environment, acting often with more or less rapidity on the organism, 

 resulting at times even in a new genus, or even a family type. Natural selection, act- 

 ing through thousands, and sometimes millions, of generations of animals and plants, 



