164 GENERAL BIOLOGY 



sperm meeting a white egg as of a white one meeting a black, and vice 

 versa. But it must not be forgotten that not only the half breeds, but 

 also the pure bloods of the dominant type will all probably look alike 

 as to color. This appearance of the same color in the half-breeds that 

 appeared in the dominant pure-blood, is the thing which confused men 

 for many years, and it was only after Abbott Mendel gave us his ex- 

 planation that we have been able to understand why this is so.f 



Mendelism has also added some interesting biological speculations 

 to the earlier idea of naturalists. 



If we define species, as meaning all those particular plants and ani- 

 mals which can interbreed and in turn give birth to fertile offspring, it 

 can be seen immediately that we cannot have any new species at all, 

 because, if the offspring of such plants or animals can give birth in turn 

 to other offspring, they belong to the same species as do their parents, 

 and if they differ in appearance from their parents they can only be 

 called variations of the parent species. If they do not interbreed, or, if 

 after interbreeding, they give birth to non-fertile offspring (such as the 

 mule, which is the non-fertile offspring of a mare and a jack), then of 

 course there can be no further offspring, and we can have no further 

 species. 



Mendelism has added a very important and interesting fact to such 

 theorizing. For example, in the dominant type of offspring, there is 

 always a pure recessive sperm and egg, so that it follows, that at any 

 time in the future, if by chance such pure egg and sperm meet, a totally 

 different type of plant or animal than its parent may be produced. But 

 this may be merely the coming forth of a plant or animal similar to 

 some ancestral form, which was the result of two recessive germ-cells 

 meeting. Therefore, although these recessive germ-cells were always 

 present in all ancestors, they were covered up in so far as external 

 characteristics are concerned by the dominant characteristics. A new 

 species, such as this which comes forth suddenly, is called a "sport" 

 in nature, and the theory that all new forms come forth in this way is 

 called the mutation theory. But, as these so-called new forms may be 

 explained as being recessives, again coming forth after lying dormant for 

 ages, there may be here no new species at all. 



fit is well to remember however, as Professor Darbishire has said, that, while on the Mendelian 

 theory we know there are such things as dominants and recessives and that unit characteristics of 

 some types are transmitted from parent to offspring, still, all the evidence we have so far is based 

 upon color of eyes, straightness or curliness of hair, color blindness, and one or two other obser- 

 vations'of this kind. It will be seen, therefore, that these are not vital, and may not be so important 

 as we have thought them. 



