118 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



been found that "elements" of one chromosome may cross 

 over to another. These "elements" have come to be 

 generally known by the name "genes." 



Various modifications in chromosomes have been ob- 

 served by different investigators. It is only the more ex- 

 treme variations, however, that can be detected. It is 

 generally admitted that these modifications are abnormal 

 in nature; they result in abnormal distribution of chromo- 

 somes in development and are of no importance in evolution. 

 Conklin says: 



Abnormalities in the distribution of chromosomes of a cell 

 persist in all its daughter cells and are not usually corrected. 

 On the other hand, abnormalities in the distribution of cytoplas- 

 mic substances to the cleavage cells frequently occur and usually 

 the cell returns to a normal condition by a process of regulation. 

 But when once the normal number of chromosomes has been 

 changed there is no regular means of restoring that number. 

 Momentary changes in the temperature, density or chemical 

 composition of the medium may thus produce permanent changes 

 in the distribution of chromosomes and of germplasm. Such 

 changes in the distribution and number of chromosomes are 

 usually of so gross a character that the development is not only 

 very abnormal but also the resulting organism is incapable of 

 continued life and reproduction. Consequently such changes 

 do not usually give rise to new races or species; only in the most 

 favorable cases, where at least the full set of haploid chromosomes 

 is present does the new form survive and reproduce. 



The central thought that has developed in connection 

 with the researches in genetics is focused on "genes." The 

 investigations that have led up to this position are too 

 numerous to review here. To those who may doubt the 

 validity of this concept, we can only refer to the extensive 

 reviews and discussions which have been published — such 

 as ConkUn's, Morgan's, Bateson's, Wilson's and others. 

 Certainly, the experimental and observational evidence 



