There is another possible mode of transition, namely, through the acceleration or retardation of the period of reproduction. This has lately been insisted on by Professor Cope and others in the United States.
Bada beste iragaitza-era bat ere, alegia, ugaltzaroa aurreratuz edo atzeratuz burutzen dena. Azkenaldion Cope irakasleak eta beste batzuek azpimarratu dute hau Estatu Batuetan.