THE LIMIT OF SPECIES. 157 



You need not be reminded that the relations of varieties to 

 species constitute to-day one of the most absorbing subjects 

 in natural history. Although we must not to-night deal with 

 this subject at any great length, it is necessary that we 

 should try to gain a clear notion of what we may call the 

 provisional limits of species. This we will endeavor to ac- 

 comi3lish by a few illustrations. Let us begin with the oaks 

 of this State, and recall a few of the facts presented in botan- 

 ical treatises. 



The scarlet-oak ^ and the red-oak ^ are described as two 

 distinct species, which are ordinarily distinguished with ease 

 from other oaks by the form of the cup and the acorn. All 

 the scarlet-oaks resemble one another pretty closely ; and the 

 red-oaks agree among each other in certain features of simi- 

 larity. Now, it has been observed also, that these points of 

 likeness are transmitted from parent to progeny; that from 

 the acorns of the scarlet-oak, scarlet-oaks are produced, from 

 the acorns of the red-oak, red-oaks grow : and from these two 

 facts of similarity, and community of descent, we derive our 

 idea of species. As matter of fact, we have only the like- 

 ness to judge from ; for of course we have not learned in any 

 one case, by direct observation, whether all the plants of one 

 sort had a common ancestry. We notice next, that, although 

 we admit that the acorns of the scarlet-oak produce scarlet- 

 oaks, there are such wide differences among the descendants, 

 that, in two cases, the varying plants have been regarded as 

 belonging to other species.^ If we have only a comparison 

 of the diiferent plants to guide us, if it is only a matter of 

 balancing points of likeness against points of difference, the 

 determination of what is a species, and what is a variety of 

 that species, must be a matter of opinion. In the case men- 

 tioned, Bartram considered the quercitron-oak a distinct 

 species; Dr. Gray considers it a variety of the scarlet-oak. 

 Michaux the younger regarded the grayish oak a distinct 

 species ; Dr. Gray looks upon it as merely a variety of the 

 scarlet-oak. Thus it is seen that there may be a difference 

 of opinion in regard to the range or limits of varieties, in 

 any special case. 



We should next observe that there are great differences 



1 Quercus coccinea, "NVangenheim. 2 Quercus rubra, L. 



8 Quercus tinctoria, Bartram; and Quercus ambigua, Michaux f. 



