VII] PLANT POPULATION 117 



they attain with a high degree of certainty to this 

 peculiar and circumscribed nidus, especially as their 

 spores are aggregated in a sticky mass. It appears 

 that Flies are the agents, and passing from station to 

 station for the purpose of oviposition, they convey 

 the sticky spores from old cultures of the Moss to the 

 fresh dung. It is hard to see how this could possibly 

 be effected without insect agency. Another case is 

 that of the fungus, Plloholns, which grows on the 

 dung of horses and cows. When matui*e it can shoot 

 off its sporangium with its sticky mass of spores to a 

 considerable distance, and they adhere to grass or 

 herbage in the vicinity. If this again be eaten by 

 horses or cows, the spores passing through the 

 alimentary tract will be ready to germinate upon the 

 excreta, and so are very efficiently dispersed. These 

 are examples of that inter-dependence which is so 

 frequently a more or less necessary condition of the 

 successful completion of the life-story. Such relations 

 will go far to determine the numbers and spread of 

 the species involved. It may thus happen that where 

 the production of germs, their dissemination, and 

 gei-mination may have all been achieved, the species 

 may still fail by reason of its inability, in some other 

 respect, to complete the cycle of its life. 



The general question which will present itself with 

 some insistence is, what positive advantages following 

 from the over-production of germs can repay the race 



