Many breeders believe in the evil effect of early reproduction. It is said that early marriage has led to the degeneracy of many nations. The domestic animals are national assets and their preservation, like the conservatives of other natural resources, is a question of vital importance to a country. But, unfortunately, this question has received very little attention among the breeders and biologists. One or two experimental stations did some work along this line, but their work is neither comprehensive nor far-reaching. Director F.B. Mumford of the Agricultural Experiment Station at the University of Missouri first conceived the idea of conducting an experiment for the determination of the proper age of animals for breeding. Unfortunately the experiment has not been more than 2 1/2 years old and data at the disposal of the writer is not at all sufficient to give any conclusive results. In the following pages, first the views of biologists and practical breeders have been given and then the results of various experiments, observations and records have been gathered. The results of the experiment at the Missouri Station have also been noted as far as possible. Finally a short criticism and discussion has been followed by conclusion.