90 THE CHROMOSOMES 



the rearrangement of a small section of a chromo- 

 some. 



The detailed analysis of evolution by cytological 

 and genetical methods has only just begun. Already, 

 however, one thing is clear — the mechanism of evolu- 

 tion (or at any rate the mechanism of species-forma- 

 tion) has not been the same in all groups of organisms. 

 Translocation and inversion of chromosome segments, 

 hybridization, auto- and alio -polyploidy are the raw 

 materials of species formation ; but they have con- 

 tributed to different degrees in different groups of 

 organisms. The mechanism of evolution has varied 

 from group to group and even from genus to genus, 

 so that it is becoming increasingly difficult to formu- 

 late general ' laws ' of evolution, and the universal 

 applicability of such ' laws ' as have been derived 

 from palaeontology and morphology is becoming more 

 and more doubtful. 



The chief limiting factor in determining the mech- 

 anism of evolution in a group of organisms is the 

 method of reproduction and the mode of sex-deter- 

 mination. At least five main types of life -cycle can 

 be distinguished, each of which probably possesses 

 a highly variable system of evolution and species 

 formation : 



1. Clonal or vegetative reproduction (Meiosis and 



Fertilization absent or non-functional). 



2. Sexual reproduction in Hermaphrodite organ- 



isms with self- or cross-fertilization. 



3. Sexual reproduction in Complex Heterozygote 



organisms. 



4. Sexual reproduction in dioecious organisms with 



a chromosomal sex-determining mechanism. 



5. Sexual reproduction in organisms with male 



haploidy. 



Species-formation in Angiosperms 



If one makes a list of the known haploid chromo- 

 some-numbers in the species of higher plants and 



