536 The American Naturalist. [July, 
two Diatoms emerges from the valves, and placed by side, but 
without conjugation, forms each an auxospore. In the fifth, 
two Diatoms divide transversely and the two halves of each 
conjugate, each half with the corresponding half of the other 
and thus form two auxospores. Before any truly natural 
classification can be made the significance of these various 
modes of producing auxospores must be understood. Whether 
the sexual or the asexual] method is the primitive one must be 
known, or whether the different methods are so many expedi- > 
ents to overcome the difficulties imposed upon these plants by 
their siliceous shells. At present our knowledge of the struct- 
ure and physiology of Diatoms is not sufficient to enable us to 
construct a perfectly natural system of classifieation, and until 
something better is proposed, Petit’s may well be adopted, for 
although it is not wholly natural, it is more so than any which 
has preceded it. 
A NEW FACTOR IN EVOLUTION. 
By J. Mark BALDWIN. 
(Continued from page 451). 
ITI. 
Social Heredity.—There follows also another resource in the 
matter of development. In all the higher reaches of develop- 
ment we find certain co-operative or “social” processes which 
directly supplement or add to the individual’s private adapta- 
tions. In the lower forms it is called gregariousnes, in man 
sociality, and in the lowest creatures (except plants) there are 
suggestions of a sort of imitative and responsive action be- 
tween creatures of the same species and in the same habitat. 
In all these cases it is evident that other living creatures con- 
stitute part of the environment of each, and many neuro-gen- 
etic and psycho-genetic accommodations have reference to or 
involve these other creatures. It is here that the principle of 
imitation gets tremendous oo intelligence and vol- 
