to form V arte lies. 



629a 



We see this in the many generic forms in a square yard of turf, and in 

 the plants or insects on any little uniform islet, belonging almost in- 

 variably to as many genera and families as species. We can under- 

 stand the meaning of this fact amongst the higher animals, whose 

 habits we understand. We know that it has been experimentally 

 shown that a plot of land will yield a greater w^eight if sown with 

 several species and genera of grasses, than if sown with only two or 

 three species. Now, every organic being, by propagating so rapidly, 

 may be said to be striving its utmost to increase in numbers. So it 

 will be with the offspring of any species after it has become diversified 

 into varieties, or subspecies, or true species. And it follows, I think, 

 from the foregoing facts, that the varying offspring of each species will 

 try (only a few will succeed) to seize on as many and as diverse places in 

 the economy of nature as possible. Each new variety or species, when 

 formed, will generally take the place of, and thus exterminate its less 

 w^ell-fitted parent. This I believe to be the origin of the classification 

 and affinities of organic beings at all times ; for organic beings always 

 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. Darwin. 



III. On the Tendency of Varieties to depart indefinitely from the Original Type. 

 By Alfred Russel Wallace. 



One of the strongest arguments which have been adduced to prove 

 the original and permanent distinctness of species is, that tarieiies 

 produced 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 distinc- 

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

 animals in a state of nature, and to constitute a provision for pre- 

 serving unchanged 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 



