TENDENCY OF SPECIES TO FOEM VARIETIES. 13 



seem to branch and sub-branch like the limbs of a tree from a common 

 trunk, the flourishing and diverging twigs destroying the less vigorous 

 — the dead and lost branches rudely representing extinct genera and 

 families. 



This sketch is most imperfect; but in so short a space I cannot 

 make it better. Your imagination must fill up very wide blanks. 



C. Dakwin". 



3, On the Tendency of Varieties to depart indefinitely from the 

 Original Type. By Alfred Eussel Wallace. 



One of the strongest arguments which have been adduced to prove 

 the original and permanent distinctness of species is, that varieties pro- 

 duced in a state of domesticity are more or less unstable, and often have 

 a tendency, if left to themselves, to return to the normal form of the 

 parent species; and this instability is considered to be a distinctive 

 peculiarity of all varieties, even of those occurring among wild animals 

 in a state of nature, and to constitute a provision for preserving un- 

 changed the originally created distinct species. 



In the absence or scarcity of facts and observations as to varieties 

 occurring among wild animals, this argument has had great weight 

 with naturalists, and has led to a very general and somewhat prejudiced 

 belief in the stability of species. Equally general, however, is the be- 

 lief in what are called 'permanent or true varieties,' — races of animals 

 which continually propagate their like, but which differ so slightly 

 (although constantly) from some other race, that the one is considered 

 to be a variety of the other. Which is the variety and which the 

 original species, there is generally no means of determining, except 

 in those rare cases in which the one race has been known to produce 

 an offspring unlike itself and resembling the other. This, however, 

 would seem quite incompatible with the 'permanent invariability of 

 species,' but the difficulty is overcome by assuming that such varieties 

 have strict limits, and can never again vary further from the original 

 type, although they may return to it, which, from the analogy of the 

 domesticated animals, is considered to be highly probable, if not cer- 

 tainly proved. 



It will be observed that this argument rests entirely on the assump- 

 tion, that varieties occurring in a state of nature are in all respects 

 analogous to or even identical with those of domestic animals, and are 

 governed by the same laws as regards their permanence or further 

 variation. But it is the object of the present paper to show that this 

 assumption is altogether false, that there is a general principle in 

 nature which will cause many varieties to survive the parent species, 

 and to give rise to successive variations departing further and further 

 from the original type, and which also produces, in domesticated ani- 

 mals, the tendency of varieties to return to the parent form. 



