38 THE PHYSIOLOGY OF REPRODUCTION 



Highlands, this number is usually limited to two. In many 

 Rodents, on the other hand, there may be six or seven, or even 

 more recurrences of the cycle within the hmits of a single sexual 

 season. 



Animals in which the oestrus does not recur during the 

 sexual season, Heape has called Moncestrous animals. Those 

 in which there is a recurrence of the didestrous cycle during a 

 single season, have been designated Polyoestrous animals. The 

 polyoestrous condition may be regarded as a device (using 

 teleological language) to increase the reproductive powers by 

 providing more frequent opportunities for successful coition. 

 But as to what factors are actually involved in bringing about 

 the rhythmic recurrence of the cycle is a question which must at 

 present be left open. 



The difEerences in sexual periodicity in both moncestrous 

 and poh'cestrous Mammals, the differences in the duration of 

 the sexual season in polyoestrous Mammals, the great variation 

 which occurs even in closely allied forms or even within the 

 Hmits of a single species, and the effects of domestication and 

 climate upon sexual and reproductive capacity are points which 

 will be considered in describing the various types of breeding 

 phenomena which exist in the different groups. 



As Heape says, " the comphcation into which an otherwise 

 simple story is thrown is due ... to variation in the quiescent 

 period." The two varieties of the quiescent period (anoestrum 

 and dioestrum) " are homologous, the one is a modification of 

 the other ; " and the modification is no doubt related to an 

 increased or decreased power of reproduction. At the same 

 time, for the purposes of the present chapter, " the difference 

 between them [must be regarded as] essential, for their relation 

 to the sexual season renders it necessary to discriminate clearly 

 between them." 



MONOTREMATA 



Little is known concerning the breeding habits of the platypus 

 and the echidna, which represent this order, the lowest of the 

 Mammaha. Semon ^ states that they breed only once a year, 



' Semon, In the Australian Bush, English Edition, London, 1899. See 

 also Sixta, "Wie junge Ornithorhj'nchi die Miloh ihrer Mutter saugen," 

 Zool. Anz., vol. xxii., 1899, 



