Nature of Heredity and Problevi of Meehamsm 315 



the hereditary attributes of the organism arc causally ex- 

 plained by the chromosomes. 



Of the many definitions the one that most marly expresses 

 the conception that will pervade tliis discussion is that oiven 

 by W. E. Castle: "liy heredity, then, we mean organic re- 

 semblance based on descent." '' The connnendahle tilings 

 about this definition are its non-commitment to any the-orv, 

 and the fact that it puts resemblance in the front line alon^r 

 with the recognition that resembhuice is chie to descint. 

 Any adequate definition of heredity nuist liold the phenom- 

 enon of resemblance always in clear sii^ht, and this in suite 

 of the fact that in the Menck'han mode of inheritance the 

 resemblance may skip one or more generations. 



Our further discussion will fall under two heads. First, 

 we shall make a wide survey of resemblance due to descent 

 for the purpose of learning how far its connection with 

 chromosomes actually extends; whether in a word, the con- 

 nection is a universal principle. Second, we shall then have 

 to see what we are justified in supposing to be the nature of 

 the connection. 



It will be noticed that the first statement admits in ad- 

 vance that to some extent resemblance between ancestors 

 and progeny is in some way connected with the chromosomes. 

 This admission relieves us of the necessity of an exhaustive 

 review^ of the evidence which necessitates the admission. 

 Though the evidence has practically all been brought (uit 

 during the last twenty-five years, it is large in quantity and 

 widely scattered. Nearly all the seml-})0})ular books, not 

 to speak of the many serials in technical biology, ])resent 

 some of it. We may therefore restrict ourselves to such 

 aspects as will serve our purpose from time to time. 



Throughout the vast range of living beings the rule like 

 produces like holds sooner or later. I say sooner or later 

 because there are many exceptions \yere we to limit the state- 

 ment to parents and their innncdiate offsj^ring. No iiidi 



