But as to our question, In what organisms did amphimixis first arise, and how? there seems, from what we have already learned with regard to the Coccidia, little prospect of our being able to give a definite answer, for if amphimixis occurs even in these lowly organisms, and occurs, too, in the same manner as in the higher unicellular organisms, and not very much more simply than among the highest multicellular organisms, we may conclude that the preliminary stages will now be very difficult or impossible to detect, either because they are extinct, or because they occur only in ultra-microscopic organisms. Nevertheless there do appear to be preliminary stages, and they are exactly those which we should have assumed if we had been obliged to construct them theoretically. The first phenomenon of this kind is the mere juxtaposition of two or more unicellular organisms, without the occurrence of fusion. This was probably first observed by Gruber in Amœbæ, and it was theoretically interpreted at a later date by Rhumbler. As many as fifty Amœbæ gather together to form a 'nest,' and remain closely apposed to each other for a fortnight. Although no fusion took place, and there were no visible results of this juxtaposition, it may be concluded that the animals had some sort of attractive effect upon each other, and it may be supposed that some sort of advantage must have been associated with this state of quiet, close apposition against one another. Cytotropism, the mutual attraction of similar cells, which Wilhelm Roux first observed in the segmentation-cells of the frog's egg, seems to occur also in unicellular organisms, and this may help us to understand how a fusion of cell-bodies may have come about.