496 TEXTBOOK OF BOTANY 



Not much more need be said to emphasize this point of view or to review 

 the initial processes of evolution enumerated in the preceding chapter. 



Heredity and heritable variations in living organisms must depend 

 upon small heritable units of matter, such as chromosomes and genes, in 

 the protoplasm. These units of matter are in the fertilized egg, and are 

 transmitted from cell to cell during cell division throughout the life cycle 

 of a plant. The smallest plants, such as the blue-green algae and bacteria, 

 have no sex, no organized nuclei and chromosomes, and yet there are 

 distinct species of them just as there are of other plants. In these smaller 

 plants the hereditary units of matter appear to be distributed throughout 

 the protoplasm of the cell. In other plants these units of matter, so far 

 as known, are mostly concentrated in the chromosomes within the nuclei. 



Changes in one or more of these units of matter may result in the for- 

 mation of new varieties. Further changes may result in what are called 

 new species. New species, then, are primarily the old species with cer- 

 tain changes in chromosome and gene complements and their consequent 

 effects on visible characters. Whenever the accumulation of aberrant 

 changes in chromosomes and genes in a particular variety of plant 

 reaches the point where the individuals of that variety in nature no 

 longer cross freely with individuals of the ancestral stock, or with indi- 

 viduals of other varieties, it is well on the way to being a new species: 

 the kind of species we recognize when we speak of red oaks, black oaks, 

 and white oaks. So long as the individuals of a variety cross freely with 

 those of other varieties of the ancestral stock, the variety fails to acquire 

 that individuality we recognize in most species, and we refer to it merely 

 as a hybrid variety. It is not always easy to decide when a variety has 

 enough individuality to be called a species. 



The origin of a new species from an old one may come about by a 

 series of changes over a long period of time, but perhaps it may occa- 

 sionallv occur rather abruptly following a rare kind of cross-fertilization 

 exemplified by the new radish-cabbage species mentioned in the pre- 

 ceding chapter. The doubling of chromosomes may also be a rather 

 abrupt beginning of a new species, particularly if the progeny with the 

 increased number of chromosomes grow in a wider range of habitats. If 

 there is no crossing with the parent stock, a few more changes in the 

 chromosomes and genes may make the variety with the double number 

 of chromosomes sufficiently different to be considered a new species. 



It is important to remember that a new species is primarily the old one 

 with a few differences in chromosomes and genes. These hereditary 

 changes in plants and animals on the earth have been occurring for a 



