The karyotypes of four species of Eulemur were known to be formed spontaneously from hybrid population of Eulemur macaco and Eulemur fulus. This research is to elucidate speciation mechanism of Eulemur macaco and Eulemur fulvus. Based on the results of from hybrid population of Eulemur fulvus and Varecia variegata. Ten metacentric chromosomes of Varecia variegata could be arranged bivalently for normal segregation at pachytene stage during meiosis. For circularly rearranged chromosomes, neighboring chromosomes of Eulemur macaco. After equilibrated gametogenesis, newly fused metacentric chromosomes could reside in the hybrid population until being fertilized with the same type of gamete. This homozygote seems to be developed in pre-macaco. It is suggested that a subsequent additional fission has occurred to give existing Eulemur macaco karyotype, meaning that F2+2 chromosome is fissioned to give F2 and 2.