No. 381.] THE CONCEPTION OF SPECIES. 685 
ered but the non-sexual tetrasporic reproduction. This is not 
an isolated case, and others will probably occur to my hearers. 
Furthermore, we must admit that the number of species nor- 
mally sexual, but in which apogamy sometimes occurs, has been 
perceptibly increased by the studies of botanists in recent years. — 
In such cases as that of Rhodymenia it may be that the cystocar- 
pic fruit really exists, and will be found later, but, since botanists 
have searched for it in vain for many years, it must be very 
rare, and certainly, as far as we know it, the plant is non-sexual. 
In regard to cases of apogamy, we have not yet sufficient data 
as to their capacity for propagating themselves continually 
apogamously, although in such cases as that of Chara crinita, 
if we may judge by the distribution of the species in central 
Europe, there seems to be no reason to believe that they may 
not do so indefinitely. The not inconsiderable number of 
species of mosses, some of them common species, in which the 
male or female only is known, and the number of marine alge, 
which, in spite of their frequency, bear only tetraspores, or at 
most bear cystocarps very rarely, should make us cautious in 
so defining what we mean by species as to imply that we con- 
sider that the perennial succession refers only to succession by 
sexual generation. 
We cannot fail to notice an increasing tendency among crypto- 
gamic botanists to give more and more weight to physiological 
characters in limiting their species. For some time we have 
been accustomed to think of the species of bacteria as largely 
physiological, and we are gradually accustoming ourselves to the 
views of those who hold the same view in regard to species of 
Saccharomycetes. More recently still we find that in another 
higher order of fungi, the Uredinacez, experts are coming more 
and more to rely on physiological characters. If in bacteria and 
Saccharomycetes we have plants which are generally recognized 
to be non-sexual, in Uredinacez the probability is that there is 
Sexuality ; at least, the probability is here much stronger than 
in the other two groups. By some the sexuality of Uredinacez 
is considered already proved, but admitting that the form of 
nuclear union demonstrated by Dangeard and Sappin-Trouffy, 
and confirmed by some other botanists, must have some impor- 
