72 JOURNAL OF THE 



into another aqnarium. They were presnmably members 

 of the first brood, the eggs of which were deposited near 

 ]\Iarch 13th. Their age, reckoning from the time they 

 were hatched, was abont 3)-^ months; size — length of shell, 

 5 mm. ; length of foot, 6 mm. In tw^o days one of the 

 snails was dead. On the 25th of July another snail of 

 about the same size was introduced from the first aquarium. 

 The next entry in my notes is under date of September 

 nth, when six nidamenta were observed attached to the 

 fibrous roots of a water plant. They were, however, small, 

 containing only from one to four eggs each, showing 

 that the reproductive fuction at that age was feeble. Some 

 of the eggs were already hatched, and the tiny grandchil- 

 dren of my first Physas were going about the aquarium in 

 search of food. Allowing, say, fifteen days for the intra- 

 capsular development of these snails of the third genera- 

 tion, I estimate that the isolated pair of the second genera- 

 tion attained sexual maturity at five months of age. The 

 same day — September nth — in the first aquarium I noticed 

 a confirmation of my observation in the second, namely, 

 the pairing of two of the oldest brood. 



The maintenance of a species depends on the equilibrium 

 between the forces tending to its destruction and those 

 tending to its preservation. We may embrace the former 

 under the general phrase, adverse external conditions. 

 There are two different ways in which the destructive 

 tendency of these adverse external conditions is opposed. 

 The first is by adaptations of structure and habit. The 

 second is by the production of new individuals to take the 

 place of those that have been overcome. Now, as differ- 

 ent animals exhibit varying degrees of ability to adjust 

 themselves to their environment, so also their reproductive 

 power may be small or great. In estimating this repro- 

 ductive power four factors, as Herbert Spencer points out,* 



i^Biology, Vol. II, p. 395. 



