T. M. SONNEBORN 207 



B. Usually, therefore, each pair of exconjugants produces two 

 clones of complementary mating types. Since the factors pre- 

 viously mentioned operate to make the animals quickly ripe for 

 mating, this will happen when complementary mating types of 

 common descent have had little time to migrate. They will there- 

 fore be apt to mate with each other, if they find mates at all. 

 Failing this, they would have little opportunity to find mates 

 other than those available from the same population. Conjuga- 

 tion with individuals that are not closely related is thus almost 

 impossible except as a rare event; conjugation with close rela- 

 tives or inbreeding by autogamy would be expected as the almost 

 universal forms of fertilization. Possibly in a sparse local popula- 

 tion provided with much food, maturity could be passed before 

 starvation occurred and induced autogamy. The same might oc- 

 cur in a sparse population with little food, as a result of the 

 difficulty of finding mates. Only in a dense population would 

 conjugation be likely on a large scale. The various characteristics 

 of these varieties would then conspire to bring about mating be- 

 tween close relatives or within the population. 



2. The extreme outbreeders, varieties 15 and 16. 3,5 These vari- 

 eties are widely dispersed from cold to warm regions of North 

 America; they have not yet been searched for elsewhere. There 

 are thus many local populations, the first prerequisite for out- 

 breeding. Most important is the fact that these are the varieties 

 with the longest known periods of immaturity, lasting for months, 

 perhaps for a year, at the low reproductive rates that may usually 

 prevail in nature. This gives time for the animals to scatter and 

 migrate. The period during which they are most likely to be 

 near close relatives is the period in which they cannot mate. As 

 a corollary, the method of mating type inheritance is less im- 

 portant when there is so long a period of immaturity. Even if 

 complementary mating types arise among the descendants of a 

 single pair of conjugants, they are not so likely to meet each other 

 when they are mature if there is an intervening long period of 

 wandering about. Actually the method of mating type inheritance 

 is unknown in variety 16; but the group A method is indicated in 

 variety 15. 



