THE CESTROUS CYCLE IN THE MAMMALIA 35 



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 complication 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 dis- 

 criminate 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 

 Mammalia. Semon l states that they breed only once a year, and 

 that in Echidna, as a general rule, only a single egg is impregnated 

 and developed at a time. After the egg is laid (for Monotremes, as 

 is well known, differ from all other Mammals in being oviparous) the 

 mother stows it away in her pouch. This is always well developed 

 at the sexual season, after which it disappears, not to appear again 

 until the approach of the next sexual season. Semon states that, 

 although the pouch is first visible in the embryo, it is thereafter lost 

 to sight until the beginning of the first procestrum. 



MARSUPIALIA 



It would appear probable that most Marsupials breed once 

 annually, but some are said to do so more frequently. Semon 2 says 

 that in the native Australian " bear " (Phascolarctus cinerevs), on the 

 Burnett, the sexual season begins at the end of October. Since he 

 failed to find pregnant females until the middle or end of November, 

 it would seem that the sexual season probably extends for three or 

 four weeks. The males at this time experience a rutting season, 

 during which they cry loudly, more frequently in the evening 

 and night, but also during the day. The gestation, as in all 



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

 also Sixta, "Wie junge Ornithorhynchi die Milch ihrer Mutter saugen," 

 Zool. Am., vol. xxii., 1899; and Caldwell, "The Embryology of Montremata 

 and Marsupialia," Phil. Trans., B., vol. clxxviii. 



2 Semon, loc. cit. 



