76 PEOCEEDIIS^GS OF THE 



a difference? No; both behave similarly during reproduction. 

 What, then, causes this difference ? Mendel has gi\'en us the 

 solution. By simple but well choseu hybridizations he has shown 

 that no individual transmits its properties as an inseparable whole, 

 but that each individual forms so many sorts of reproduction-cells 

 as there are possible constant combination forms. Consequently, a 

 pure self-fertilized individual forms but one kind of reproduction- 

 cells, by whose combination but one kind of descendants, all 

 identical with the parent, can arise, while a hybrid self- fertilized 

 individual forms different kinds of reproduction-cells whose 

 combinations cause the descendance to be a motley crowd. 



As a hermaphrodite indiA'idual, of course, i-epreseuts two parents 

 in one, there is no reason why there should be a fundamental 

 difference between the offspring of self- fertilized hermaphrodites 

 and the offspi'ing of a separate male and female. Indeed, tliere 

 is not. If the male and female are pure and belong to the same 

 type, their offspring is homogene ; if they are hybrids, or because 

 they belong to differeut types together form a hybrid, their 

 offspring is heterogene. 



AVe may therefore conclude: The descendance of two j^ure indi- 

 viduals of the same hereditarij ti/pe is as homogene as the descendance 

 of a sinijleself-fertdized pure individual, and ahsolately identical with 

 its parevts as far as hereditary piroperties are concerned. Or, in 

 other words, pure individuals and their progeny are distinct, 

 permanent, hereditary, and irreducible. But this is exactly what 

 Jordan said already iu 1873 of his micro-species. 



In fact, the two conceptions are identical ; but our present 

 knowledge of heredity permits us a better deliuition of the small 

 species, viz. : — 



A species is the total of all individuals of the same hereditary 

 composition, forming hut one hind of reproduction-cells. 



Or, in the language of Mendelism : A species is the total of all 

 homozygous individuals of the same genetic constitution. 



That, according to my view, is a species. 



We must now try to answer the question, How do new species 

 originate ? 



Conceivable are three different ways : — 



(a) By acquisition of new characters and transmission of 



these to their progeny. 



(b) By so-called spontaneous variation or mutation. 



(c) By crossing. 



The first two possibilities can be brought under the heading 

 Variability. We will therefore ask : — 



Does hereditary variability of any hind really exist'? 



We have seen already that variability does exist. The indi- 

 viduals composing a species are never alike, but only similar. 



