GENES IN CONTROL 39 



Whole chromosomes with their long complement of in- 

 dividual genes undergo changes by doubling or tripling, 

 and so on, the basic number of threads (diploidy, triploidy, 

 etc.). Such wholesale addition of chromosomes is like 

 adding more decks in a game of cards, as in canasta. This 

 type of hereditary change has sometimes been of value to 

 man in his food plants (wheat, for example), and it is al- 

 ways an additional factor in producing variation. Sometimes 

 a part of a chromosome is transferred to another part of the 

 same chromosome or even to another chromosome. Finding 

 themselves in a different allelic situation, the blocks of genes 

 involved can produce very marked changes in the organism. 

 At other times there is an inversion of a block of genes 

 when the chromosomes are paired, a condition that again 

 brings new reactions of the genes into play. Also, at times, 

 an irregularity can occur when a chromosome is added, 

 making a trisonic combination. There are indications, too, 

 that sometimes chromosomes are joined; and frequently the 

 paired threads are interchanged (cross-over) at the time of 

 germ cell formation. All these chromosome changes or ab- 

 errations, along with basic mutation, have been effective in 

 evolution, making the theoretical possibilities for variation 

 practically unlimited. The endless variety of life has been 

 made necessary by the endless means for change. In the 

 literature of biology today some 1,000,000 species of ani- 

 mals and about 250,000 species of plants have been de- 

 scribed; perhaps this is half the total number now existing. 



Sexual reproduction is the agency by means of which the 

 hereditary complex of a population can be thoroughly 

 shuffled in each generation. The union of germ cells to pro- 

 duce offspring from two different individuals has been a 

 tremendous factor in evolution. Without sex, in fact, life 

 on this earth would still be at levels hardly higher than 

 worms. We know that sexual reproduction evolved slowly 

 and finally became nearly universal among both plants and 

 animals. In this process offspring receive the chromosomes 



