ABSTRACT By polyploidy is meant an increase in the number of sets of chromosomes in a plant or animal. The bodies of animals and of the higher plants normally contain two sets of chromosomes, one derived from each parent. In ordinary sexual reproduction this number is reduced to one half in the maturation of the germ cells, and the diploid or 2x number is restored in the process of fertilisation by the union of two germ cells whose nuclei are each carrying the haploid or x number. But the comparative study of chromosomes shows that there are many instances in plants and animals where additional sets of chromosomes have been acquired by the organism, thus giving rise to species, genera, or individuals which are triploid (3x), tetraploid (4x), pentaploid (5x), hexaploid (6x), or even decaploid (10x) in their chromosome outfit. These conditions of polyploidy have a number of interesting consequences and evolutionary bearings which it is the purpose of this article to discuss.