The Practical Conception of Species. 589 
can hope to arrive at a conception of species which shall 
be both in accord with the facts, and justified by experi- 
ment. 
Of course, I am well aware that the experiments 
which have been carried on up to the present are by no 
means sufficient and that much remains to be done. Nu- 
merous experiments in hybridization are necessary before 
they can serve as a foundation for systematic distinctions. 
But the leading principle in these researches must always 
be the attempt to determine the elementary characters. 
4. THE PRACTICAL CONCEPTION OF SPECIES. 
Both collective and elementary species are called spe- 
cies ; and this twofold significance of the word has thrust 
its roots so deeply into the history of descriptive biology, 
that it will probably never be wholly eradicated LIN- 
NAEUS himself confused the two ideas; and whilst some 
readers derive from the study of his works the conviction 
that in his mind the collective forms were the true spe- 
cies, 1 others come to a different view of his attitude, and 
believe that in formulating the conception of species, he 
was considering the real units of the system. 2 
The fundamental conception from which almost all 
investigators start, is that species are the only real enti- 
ties. 8 As to what these entities are, opinions differ. 
"On ne peut pas douter," says DE CANDOLLE, "que le 
groupe appele espece par 1'illustre Suedois ne fut, dans 
1 See Vol. I, p. 20. 
~ S. BELLI, Observations critiques sur la rcalitc dcs espcces en 
nature au point de vue de la systematique des vcgctaux, 1901. 
3 C NAGELI, Entstchung und Bcgriff der naturhistorischen Art, 
1865, p. 31. 
