272 



THE AMERICAN NATURALIST 



[Vol. LIV 



A brief life cycle is extremely important because numerous 

 generations must be raised in order to secure adequate data for 

 the analysis of more complicated genetic problems. In this 

 respect, no sexually propagated flowering plants can compare 

 with the insects. On the other hand, certain highly desirable 

 features possessed by plants are either impossible or very diffi- 

 cult of realization in animals. For example, asexual reproduction 

 can often be resorted to in plants when it is desired to per- 

 petuate a particular individual for comparison with later gen- 

 erations. But the most important point of superiority of plants 

 over insects for genetic study is the greater possibility in plants 

 of securing hybrids between different species. That this advan- 

 tage should receive considerable weight will be admitted by all 

 who recognize the need of studying hybrids between species hav- 

 ing different chromosome numbers. The desirability of such 

 investigations has been mentioned recently by Morgan (1919) 

 as follows: 



The theory that the chromosomes are made up of independent self- 

 perpetuating elements or genes that compose the entire hereditary corn- 

 species have an immense number of genes in common, makes the numer- 

 ical relation of the chromosomes in such species of unusual interest. 

 This subject is one that could best be studied by intercrossing similar 

 species with different numbers of chromosomes, but since this would 

 yield significant results only in groups where the contents of the chromo- 

 somes involved were sufficiently known to follow their histories, and since- 

 as yet no such hybridizations have been made, we can only fall back on 

 the suggestive results that cytologists have already obtained along these 



I have italicized one clause in the above paragraph in order 

 to emphasize the importance of extensive genetic analysis in those 

 particular species which are to be used in intercrossing exper- 

 iments. It is not sufficient that the species have low numbers 

 and different numbers; it is also necessary that the inheritance 

 of a sufficient number of characters in each species be studied so 

 as to establish the linked groups of characters or genes corre- 

 sponding to the chromosomes of each species. Only then can the 

 contents of the chromosomes involved be sufficiently known to 

 follow their histories in the hybrids. 



Thus we find several excellent reasons for seeking among plant 

 materials for a group of species which possess as many as pos- 



