yoS THE ORIGIN OF SPECIES 



and the redbreast. These are now separated into half a dozen 

 genera, and Motacilla is used only for the wagtails. This may 

 necessitate the change of an animal's generic name without a 

 change in the trivial. A name so altered is shown by following 

 it with the name of the author responsible for the trivial name 

 enclosed in brackets. An author's name (or a standard abbrevia- 

 tion of it) without brackets shows that the whole specific name 

 was given by him. For example, the redbreast is Erithacus 

 nihecula (L.), because Linn^us used the trivial nihecitla, but the 

 generic Motacilla ; while the pied wagtail is Motacilla alba L. 



In much-studied groups of organisms, especially birds and some 

 orders of insects, it is convenient to recognise groups within the 

 species, and these, called subspecies, are distinguished by tri- 

 nomials, a second adjective being added to the specific name. 

 For example, the pied wagtail of Great Britain is Motacilla alba 

 yarelli, while the Continental form, the white wagtail, is Motacilla 

 alba alba. The system of application of authors' names to tri- 

 nomials is in confusion, but fortunately they are of no importance 

 to the elementar}^ student. 



The further process of classification consists in arranging the 

 genera into series of groups, each containing more and more 

 species which have less and less in common. The chief categories, 

 in ascending order from the genus, are families, orders, classes, 

 and phyla, but other categories, such as sub-families and super- 

 orders, are often interpolated to make it easier to manipulate 

 large numbers of species. All these groups have Latin names, 

 which are plural nouns. Family names are derived from generic 

 names by the addition of -idcB to the root, for example, Felidae, 

 the cats, and Motacillidae, the wagtails and pipits, but there is no 

 rule for the others. Group names are not italicised. Anticipating 

 what comes below, we may add that it is now customary in 

 classifying animals to attempt to show not only the features 

 which they have in common with each other, but also the genetic 

 relationships which they may be supposed to have according to 

 the hypothesis of organic evolution. 



The existence of species poses the problem of their origin, 

 and, to solve this, three main hypotheses have been proposed. 

 The first, that species have always existed as they are now 

 without beginning, has been held by few educated people, and 

 all that we know of the history of the earth is against it. It has 

 been attributed to Aristotle and to Duns Scotus (1265 ?-i3o8) 



