PROTOZOAN FAUNA OF HAY INFUSIONS 259 
his data show that the succession of those forms is not deter- 
mined by the kind of available food but to an unused increment of 
‘decomposition and excretory materials which, in the last analy- 
sis, affects breeding.” 
5. Decline in numbers 
Closely involved with the problem of the sequence of appear- 
ance and maximum of the various forms is that of their more or 
less rapid decline in numbers. Here again the accumulated data 
do little more than establish the fact. The decline of the monads 
is quite clearly due, in part at least, to the evident variation in the 
amount of food in solution and to the rising hosts of Colpoda. 
The decline of Colpoda may be similarly ascribed to the domi- 
nance of the hypotrichida. Most of the hypotrichous forms were 
literally filled with ingested Colpoda which formed their staple 
diet. The relations of Paramaecium and Vorticella to their pre- 
decessors, successors and to each other is not so apparent, but 
their abundance may well be influenced by a succession of the 
bacterial flora, for example, which unfortunately could not be 
followed in these studies, as well as to the host of other protozoan 
species. The competition between the various forms is so keen 
and the cycle is so rapid that even daily observations are, at times, 
insufficient to reveal the kaleidoscopic changes. Now and then, 
however, some prominent case of competition, such as that be- 
tween Paramaecium and Didinium, is forced upon the attention 
and the reason for the extinction of one form is clear. Didinium, 
in fact, so quickly exterminated the paramaecia in groups A [[ 
and B IV that it was necessary to omit the records of paramaecia 
in the table of sequence of these infusions (cf. table 2). In B I 
also the paramaecia cycle was considerably aborted by Didinium. 
Among other instances of a similar nature, the destruction of hosts 
of Colpoda by the suctorian Podophrya may be mentioned. In 
other words, one who closely follows a series of infusions dayby 
day cannot but be impressed with the intense struggle for food 
and the eternal warfare in this microcosm, and become con- 
17 Biological Bulletin, vol. 22, no. 1, 1911. 
