ROTATORIA OF THE UNITED STATES. 
69 
of the greatest interest, leading to the general question of the origin and nature of 
species and to the deepest questions of morphogenesis. The variations of a given 
species should be recorded and studied; from a sufficient number of records the laws 
of the variations may be deduced. More important still is a study of the causes of the 
variations. The variations occurring with a change of conditions, or with a change of 
season, are worthy of careful study, such a study as has recently been made by Lauter- 
born (’98) of Anurcea cochlearis. An experimental study of the production of variations 
promises most for this problem; to this end the very variable and hardy species 
Brachionus bakeri would probably be a most useful form. 
The life-history and reproductive habits of the animals furnish a further important 
field for study. Many features in the life-history of the Rotifera are of much interest 
and need such study as has been carried on thus far only for Eydatina senta, by Maupas, 
Tsnssbaum, and others. The striking sexual dimorphism ; the conditions of the occur- 
rence of males at certain periods; the determination of the different kinds of eggs pro- 
duced at different times; the sex of the offspring — all present problems of the greatest 
interest for an experimental study. 
A most important and neglected field lies in a study of the activities by which 
the animals respond to their environment. Proper conditions of existence are neces- 
sary, but these are not sufficient alone to preserve the life of the organisms ; the animals 
must respond to their environment by appropriate activities. We must know what 
these activities are and the laws which govern them. In other words, a study 
of the physiology and especially the psychology of the animals is necessary before we 
can understand the interaction of organism and environment; the functions and move- 
ments of the organisms constitute one of the chief factors in the network of interrela- 
tions of the “ organs ” of the lake. Having determined the general laws according to 
which the organisms respond to their environment — in other words, having made a 
study of the psychology of the animals — it will then be possible to determine by obser- 
vation and experiment the specific factors which cause migrations, the sudden appear- 
ance of the animals in a given locality, their quick disappearance from another locality, 
and the like. In any group of animals the investigation must follow some such line 
as marked out above, in each case a study of the normal psychology of the animals 
being a prerequisite to an understanding of the laws of their migrations and other 
striking activities. Commencing at the beginning, a study of this nature was made 
on the Protozoa during the summer of 1898, the results of which are resumed in 
another paper, which deals with the Protozoa; it is hoped soon to extend this study to 
the Rotatoria. The psychology of the Rotatoria has been studied scarcely at all; 
notes as to the nature of the food and the method of taking it, together with descrip- 
tions of the method of forming the tube in some tube-dwelling species, being the chief 
matters that can be gleaned from the literature. 
The only way in which the problems above characterized can be solved, the 
relations of the Rotifers (or of any other group) to their environment worked out, is 
for investigators to choose definite limited problems for solution and devote time and 
energy to observation and experimentation till the questions proposed are answered. 
Mere isolated observations, collected during a systematic study of the group, can do 
little; investigators must take up the work in the same spirit in which morphological 
jjrobleins are attacked, concentrating all efforts upon a given point until that is 
settled. The activities of animals are as worthy of such study as are their structures. 
Until a large amount of investigation has been done it will not be possible to give any 
