DIV. I MQRPHOLOGY 207 



tion, morphology, the geographical distribution of plants and animals, 

 and by palaeontology. 



1. EVIDENCE FROM CLASSIFICATION. According to the theory 

 of special creation the various species of plants were created inde- 

 pendently and are essentially constant. They were supposed to be 

 so little subject to change that one species could not arise from 

 another ; at most a species could give rise to more or less inheritable 

 varieties. This view thus assumes that there are sharp limits between 

 the species, and also that there is an essential difference between 

 species and varieties. As the student of classification proceeds to 

 examine any group of organic forms he finds that there are no 

 characters to be relied on to distinguish varieties from species. The 

 amount of morphological difference between the species of a genus, 

 the varieties of a species, or between species and varieties, is quite 

 undetermined. It has also come to be recognised that species are 

 not independent morphological units but in many cases are compre- 

 hensive groups of forms or petites espkces (e.g. in the genera 

 Erophila, Rubus, Bosa, Hieracium, Quercus). The sharp differentiation 

 of such species from other species, i.e. other groups of forms, is 

 frequently difficult or scarcely possible. The constant small species 

 often differ less than do many so-called varieties. It thus becomes 

 a matter of taste or "systematic sense" whether a particular form 

 should be regarded as a species or a variety and how a species should 

 be delimited. The rule formerly relied upon, that crosses between 

 two independently created species would be sterile while those 

 between two varieties of a species would be fertile, has proved 

 untrustworthy ; fertile and sterile hybrids are known both between 

 two varieties and two species. There are not only transitions 

 between species but between genera and even families, so that in 

 these cases also the limits have to be drawn at the discretion of the 

 systematise All these facts only become comprehensible if it is 

 assumed that species were not independently created but are capable 

 of heredity with variation, so that new species can be derived from 

 others by inherited changes, while more marked changes give rise to 

 new genera or families. On any other assumption it remains incon- 

 ceivable why organisms can be placed in groups of lower and higher 

 order (species, genera, families, classes, etc.), which are in part co- 

 ordinate (like the species of a genus or the genera of a family) and 

 in part subordinated to others (like the species to the genus or the 

 genera to the family) ; further, that the groups of extinct organisms 

 which lived in earlier geological periods can as a rule be naturally 

 placed in the same classification as the existing forms. All these 

 difficulties disappear when organisms are regarded as blood relations, 

 and the natural system as expressing their nearer or more distant 

 relationship, and thus, in a certain degree, as a genealogical tree of 

 living beings. 



