EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 341 



of Mendelian segregation and its implications. Though the corre- 

 sponding facts have not yet been recorded for the lower organisms 

 with the same detail as for the higher, there is reason to believe that 

 they are of general application wherever sexuality exists : for the 

 details so far observed in the lower resemble closely those for the 

 higher forms. The conclusion may then be applied generally that 

 the sexual cells are the bearers and the distributors of hereditary 

 characters. 



But there are many simple organisms, often unicellular, in which 

 no specialised phenomena of sex are known. They may show a 

 multiplicity of forms ranked as genera and species, notwithstanding 

 that they are sexless. This suggests that the absence of a specialised 

 mechanism of heredity does not entail an absence of variation, nor 

 even of the hereditary transmission of characters. Probably such 

 simple organisms as these are really primitive. Sexual repro- 

 duction developed as a later condition and brought with it various 

 advantages : among the most important is the orderly distribu- 

 tion of the hereditary characters (Chapter XXXV.). It thus appears 

 that a Theory of Evolution is of general application to organisms 

 higher or lower in the scale, and that it finds its true basis in the 

 facts of variation. An essential feature is that certain structural 

 variations are heritable. By Natural Selection these are sifted out, 

 perpetuated, and even accumulated. Other factors may also be at 

 work : but these are commonly held to be the most important. The 

 central problem of Evolution will therefore lie in the origin of those 

 variations that are heritable : while the mechanism of their transmission 

 to, or distribution among, the offspring naturally takes its place as 

 accessory. 



Using some such evolutionary theory as a working basis, a 

 succession of organisms may be passed in review, starting with those 

 that are relatively simple and proceeding to those that are more 

 complex. But in doing this it is necessary to remember that Evolu- 

 tion has not been a simple matter. Organic life has progressed along 

 a great number of distinct Lines of Descent, which may have been 

 divergent, parallel, or convergent as regards their characters. More- 

 over, while the progressions which may be traced will in the main be 

 from simpler to more complicated states of organisation, the converse 

 may sometimes be the case. It has been seen in Chapter XII. how 

 parasitism in Flowering Plants, which in part or in whole relieves 

 them of the function of self-nutrition, often leads to a reduction 

 of the vegetative system. It has been noted that Cuscuta has 



