176 THE INFLUENCE OF INANIMATE SOKEOUNDINGS. 



the dry mud. Living specimens of Apus caught by me in 

 Wurzburg deposited a large number of eggs in the water of 

 my aquarium ; not one developed. Mud full of eggs from 

 the same pool from which I had taken the fully-grown Apus, 

 after it had lain by for a full year, stUl yielded no larvae on 

 being wetted, and it was not till the second year that I ob- 

 tained a few; but from that time I succeeded regularly in 

 mating them develope in great numbers, and whenever I chose. 

 The statements of Brauer and others coincide with this. The 

 eggs of the BrancMpus, so nearly allied to Apus, do not share 

 this peculiarity, but develope equally well whether they have 

 been dried or kept constantly in damp mud. Brauer points 

 out indeed, in his interesting notes on his experiments in breed- 

 ing, that it would be very easy to rear animals of these groups 

 from different parts of the world in our laboratories, and to 

 study them at our convenience ; since nothing would be needed 

 but to obtain some dried mud from the localities where they 

 live. In this way, for instance. Professor Glaus was recently 

 enabled carefully to investigate, in Vienna, the anatomy of the 

 beautiful Daphnia Atkinsoni from Jerusalem. It would cer- 

 tainly be a grateful task to determine exactly what species of 

 animals have eggs which can endure desiccation, or even abso- 

 lutely require it, like Apus, to qualify them for development, 

 and to find out also what the maximum period is during which 

 they can endure to lie dry without injury to their vitality. A 

 fundamental investigation of these questions would undoubtedly 

 contribute much to a. satisfactory explanation of many peculiar 

 facts in the geographical distribution of the lower animals.^* 



Concluding remarks.^ — If we now compare the facts 

 established in the different sections of this chapter with those 

 previously ascertained, we obtain again the same general laws. 

 Animals living in the same places, and apparently under the 

 same external conditions of existence, nevertheless behave in 

 quite different ways under the influence of the various sub- 

 stances held in solution in the water, as salt, oxygen, carbonic 

 acid, <fec. The ova of different and yet very closely related 

 forms can endure a long period of drought, or even require it 

 to enable them to develope. Hence every change, as, for in- 



