xi. c, 4 Copeland: Natural Selection 157 



groups will continue to be recognized. As we fill in the finer 

 details, suppose that we recognize the twigs as species, shall 

 we recognize the leaves as well? The leaves are parts of the 

 whole structure, but are transient in their nature. Eventually, 

 the most of each season's twigs, on each individual tree, also 

 die and fall. Of the species that occupied the world five million 

 years ago, some may survive, but the most have doubtless dis- 

 appeared. Working in any particular age, as we have the 

 opportunity to work in our own, we may identify each recog- 

 nized twig of the tree of life as something that for our purposes, 

 extending over a period of generations, is established, and we 

 may give it a specific name. But, again, what shall we do with 

 the leaves, of which each season perhaps yields its own crop? 



To my mind, this figure of the tree of life is appropriate and 

 useful. Granted that we retain the finest twigs as species, the 

 leaves are each season's crops of variations. As the morpholo- 

 gists distinguish easily in general between leaf and twig, so the 

 systematic botanists can distinguish in a general way, although 

 with less accuracy and with more difficulty, between the fluc- 

 tuating variations and the recognized species, which hold their 

 own through the time with which we are acquainted, and are 

 widely scattered. Yet, there is no sharp line between these. 

 Of each season's crops of variations, the most disappear in their 

 turn as regularly as they present themselves ; yet of each season's 

 crop, some variations in form perpetuate themselves for a longer 

 or shorter time — some for one more generation, some for several 

 generations, and so on up to what in a practical sense we refer 

 to as "forever." There is no sharp line between the most tem- 

 porary variations and the most lasting. And in selecting among 

 the plants that manifest these forms of varying distinctness and 

 durability those which we will recognize as species, convenience 

 is the only ultimate criterion that can possibly guide us. 



In each locality, each species produces its own crop of varying 

 offspring. In each season, each species that occurs on a number 

 of islands or a number of mountain tops produces in each locality 

 its crops of variants. Some of the variations are so slight as to 

 escape any attention that they might receive. Others, the visit- 

 ing or resident botanist notices, but ignores as of little impor- 

 tance. Others are more striking. If they impress him as suffi- 

 ciently marked, he describes a new species. What constitutes 

 sufficient markedness depends on the idea of propriety held by 

 the individual botanist. Visiting one of our mountain tops, 

 Linnaeus would have found a few species ; Jordan, a very large 



