THE PROBLEMS OF HEREDITY AND THEIR SOLUTION. 569 



believe, oc'iH'r;illy coiisidered that the law of aiiceslral heredity was 

 to he taken as :i[)plyiiijj- to phciioineiia like these (coat eolor, eye color, 

 etc.) where the inheritance is i;-enerully alternative, as well as to the 

 phenomena of blended iidieritance. 



Pearson, in the writings referred to, besides withdrawino- other 

 large cateo-ories of phenomena from the scope of its operations, points 

 out that the law of ancestral heredity does not satisfactoi'ily express 

 the cases of alternati\'e inheritance. He urges, and with reason, that 

 these class(\s of jjhenomena should be separately dealt with. 



The whole issu*^ as regai'ds the various possibilities of heredity now 

 recognized will be made clearer l)v a \ ei'v brief exposition of the 

 several conceptions involved. 



If an organism [)roduciiig germ-cells of a given constitution, 

 uniform in resp(H't of the charactei's they bear, breeds with another 

 organism " hearing pi-ecisely similar gei-m-cells. the otfs})ring resulting 

 will, if th(> conditions are identical, he uniform. 



In i^ractice such a j)henomenon is seen In pure breeding. It is true 

 that we know no case in natur(^, where all the germ-cells are thus 

 identical, and where no variation takes place l)eyond what we can 

 attribute to conditions, l)ut we know many cases where such a result 

 is appioached, and v(U'v many where all the essential features which 

 we regard as ct)nstituting the characters of the breed are reproduced 

 with approximatt^ certainty in every member of the pure-bred race, 

 wdiich thus clos(dy api)r()ach to uniformity. 



But if two germ-cells of dissimilar constitution unite in fertilization, 

 what oifspi'ing are we to expect^'' First let us premise that the 

 answer to this question is known experimentally to differ for many 

 organisms and for many classes of characters, and may almost cer- 

 tainly he in part determined by external circumstances. But omitting 

 the last (lualitication, certain j)rinciples are now tdearly detected, 

 though what principh^ will apply in any given case can only be 

 determined hy direct experiment made with that case. 



This is th(^ plHMiomenon of cross breeding. As generally used, this 

 tei'm means th(^ union of nuMubcrs of dissimilar vai'ieties, or species; 

 though when dissimilar gametes'' produced hy two individuals of the 

 same variety unite in fertilization, we have essentially cross breeding 

 in res|)ect of the character oi' characters in which tliost^ gametes 

 diii'er. We will suppose, as before, that these two gametes bearing 



a For gimj)lic'ity the case of .self-fertilization is omitted from this consideration. 



& In all the cases discussed it is assumed that the "jametes are similar except in 

 regard to the "heritage" they l)ear, and that no original variation is taking place. 

 The case of mosaics is also left wholly out of account (see later). 



''The term "gamete" is now generally used as the e(|uivalent of "germ-eell," 

 whether male or female, and the term " zygote " is here used for brevity to denote 

 the organism resulting from fertilization. 



