10 INTRODUCTION. 
corresponding series of plants of the Southern United States. Upon 
bringing them together, he would find that many species are common to 
the two places; but that some are peculiar to the Northern United 
States, others to the Southern United States. If he obtained, in addition 
a collection of South American plants, he would find that some of the 
species common to the Northern and Southern United States, are con- 
tained in it also; and that some species not known in the Northern 
United States are common to the Southern United States and South 
America; but he will find many peculiar to South America. Proceeding 
thus over the whole world, he would gradually increase his number 
of new species; at the same time adding considerably to the number of 
specimens of some which he would find very extensively diffused. He 
would find a few similar species almost every where,—these being the 
kinds most capable of adapting themselves to varieties in soil, climate, 
&c.; whilst on the other hand, he would find many of a very limited 
distribution,—being restricted to some small extent of country, in which 
alone they can find the conditions necessary for their growth. . 
DISTINCTION BETWEEN SPECIES AND VARIETIES. 
3. The greatest difficulty in this part of the investigation consists in 
the discrimination of species really distinct,—that is to say, of races which 
have maintained their distinctive peculiarities, so constantly, that they 
must be considered as haying had originally different stocks,—from 
those varieties, which may often present differences really greater in 
species,.but which all sprung from the same original stock. Thus, for 
example, a collection of plants from different parts of India, would 
contain many specimens presenting such marked differences that the in- 
experienced Botanist would not hesitate to set them down as distinct 
species ; yet to one who has carefully examined the subject, and has made 
himself acquainted with the variations produced by the differences in soil 
and climate, so striking in this extensive tract, it becomes apparent that 
they are all members of the same. There is, too, in many species a 
remarkable tendency to run into spontaneous variations, for which no 
external influences will account. Thus the seeds of the same individual 
of the beautiful Fucusta, now naturalized in our green-houses, and in — 
the open air of the milder parts of the Northern United States, have 
been known to produce plants, whose flowers differ so much in shape 
and in the proportional length of the calyx and corolla, that if these had 
been collected and compared without the knowledge that they had been 
produced from one plant, they would have been regarded as distinct 
species, perhaps even (so striking is the difference) as distinct genera. 
Nearly the same is the case with another South American Plant, now 
