( 433 ) 



illustration of the fact that in a certain district a transformation of a species takes place 

 has been published during recent years from different sides. A moth of the family 

 Geometridae, Amjjhidasis heiulariv.s, has a black and a white form (with intergrada^ 

 tions) : the black form (douhledayamus), which for a long time was known to occur 

 in Great Britain, was very rarely found in North- Western Germany ; during the last 

 ten to twenty years, however, the black form has become more frequent, and is at the 

 Lower Khine now nearly as common as the white form. Without tr^'ing to give an 

 e.xplanation of this phenomenon, which speaks for itself, it is of interest to note that 

 in countries with a rational cultivation of the soil the changes in the immediate 

 neighbourhood of the animals are very varied : the disapjiearance of swamps, planting 

 of new forests on barren hills, artificial watering of pasture land, and so on, will not 

 only destroy many forms, and give opportunity for other forms coming into the 

 district ; but these changes will most probably also have a certain amount of influence 

 on those animals and plants which remain residents, at least on those forms which, 

 like butterflies and plants, are very sensible to a change in the biological factors ; 

 and it would be highly interesting to have correct data whether perhaps a visible 

 transformation of some animals and plants would be effected in a shorter time in a 

 country with an intensive cultivation of the soil (like France) than in a country (like 

 J^nglaud) in which agriculture is more stagnant. Certainly we will not maintain that 

 all the forms of animated nature have changed during the last hundred and fifty 

 years, but will only draw attention to the historical side of the question, which 

 cannot be entirely neglected. Palaeontology teaches us that there are forms which 

 for a very long time have not changed, at least not in those parts which have been 

 pi-eserved ; while, on the other hand, we know from palaeontology that in most cases 

 a change must have taken place. 



At subsequent geological epochs the earth has been inhabited by different 

 faunas and floras ; that we know. If we had now before us all the specimens of 

 animals and plants which ever have existed, and tried to group them as we do with 

 the specimens in our collections, what would be the result ? The gaps between the 

 various forms would all be filled up bv intergradations ; * the Gorilla would be as 

 different from the Giraffe as he is now; but tracing both animals back to their 

 common ancestral form, we could draw up a series of individuals which would perfectly 

 connect the two animals, and which would not allow us to draw a line of division : 

 Gorilla and Giraffe thus would appear to be merely the extremes of the series. If, as 

 • we mostly iiiactically do, the " species " are based only upon morphological differences, 

 all the animals and plants would belong to one variable species. Any definition, 

 therefore, based solely upon morjihological differences must necessai'ily be a failure, and 

 it follows also that the allegation that a certain form is a " distinct species " because 

 there are no intergradations between it and the allied forms is in discordance with 

 the theory of evolution, according to which the intergradations did occur-, but are 

 perhaps now extinct. 



If we trace the line of ancpstrals of two given types back to the form from which 

 both have descended, wo have for each type a separate line until we come to the 

 primordial type in which both lines combine. The line of ancestors of every species 

 comprises the different steps of development from the primordial form down to tlie 

 recent form, and represents the historical polymoqihism of the species, in opposition 

 to contemporary polymoi-phism. Are the differences between the steps of develop- 

 ment " specific '" differences ? Hipparion and Eqmcs, though assumed to stand in 

 * Koinanes, Darwin and After Darwin TI. London, 1895. |). 282. 



