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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 
