1074 The American Naturalist. [December, 
impressed with the amount of difference in the forms 
which he is continually studying; and he has little general 
knowledge of analogical variation in other groups and in other 
countries by which to correct his first impressions. As he ex- 
tends the range of his obser\^ations he will meet with more cases 
of difficulty, for he will encounter a greater number of closely al- 
lied forms. But if his observation be widely extended he will in 
the end generally be able to make up his own mind ; but he will 
succeed in this at the expense of admitting much variation, — and 
the truth of this admission will often be disputed by other natur- 
alists. Where he comes to study allied forms brought from 
countries not now continuous, in which case he cannot hope to 
find intermediate links, he will be compelled to trust almost en- 
tirely to analogy, and his difficulties will rise to a climax." 
It would be an impossibility, even were it desirable, to refer to 
all the recorded cases of variation among animals and plants. 
But it will be instructive to turn to some authorities in diflfierent 
departments, and listen to what they have to say on this subject. 
Dr. Isaac Lea was well known as a describer of species, but he 
has put upon record his opinion of variation in the great genus 
Unio. It is well to note here that he attempted to divide the 
genus into different genera, but gave it up finally as impracticable 
and useless. In a paper read before the American Philosophical 
Society on November 2, 1827 (p. 260, as published in the 
" Transactions ") he says : 
" It has been doubtful with some concologists whether the 
species of the genus Unio are not the mere varieties of one spe- 
cies. To the naturalist, who has the opportunity of examining 
numerous specimens, the gradations are so interesting, and at the 
same time so perplexing, that he is lost in the maze of their 
changes, and he seeks almost in vain to draw a distinctive line 
between them ; for even the tuberculated shells sometimes pass 
by almost insensible gradations into smooth ones." 
In another paper read two years later (American Phil. Society, 
read March 6, 1829) he says: "The number of species [of 
Unio] adds greatly to the difficulty of distinguishing them, for 
they glide into each other so insensibly through their varieties 
