2 22 POPULAR SCIENCE MONTHLY. 



chromosomes this does not prove that the chromosomes cause the other 

 differences, even though the differences of the chromosomes inter- 

 fere with the conjugation of the reproductive cells and thus prevent 

 the hybridization of the plants. Species or varieties seldom, if ever, 

 differ by single characters or at one stage merely, and there is no 

 known reason why related species should not diverge in their single- 

 celled condition as well as at any later period. It is rapidly becom- 

 ing apparent that the internal organs and functions of cells are as 

 diverse as those of embryos and adult organisms, and as much in 

 need of a general evolutionary explanation,* 



The notion that heredity, variation or other phases of evolution 

 are the functions of special organs or mechanisms of cells, has no 

 ascertained basis of fact, and is but an inference from the traditional 

 evolutionary errors that species are normally constant or stable, and 

 that developmental changes are the results of external influences. To 

 move a stationary organism some sort of 'hereditary mechanism' would 

 be needed to bring about the inheritance of characters 'acquired' from 

 the environment, but if we consider that the individuals of a species 

 are normally diverse, and that the species as a whole is normally in 

 motion, a 'hereditary mechanism' becomes quite superfluous, or may 

 be identified with the organism itself, whether in a unicellular or a 

 polycellular stage. 



Heredity is the term under which we allude to the fact that organ- 

 isms exist in series of similar individuals; we have as yet no warrant 

 for holding that it is special 'force' or agency. Crystals of the same 

 substance are thought of as repeatedly taking the same form because 

 of certain properties of matter, not because of a special crystallizing 

 mechanism. The analogy of crystals is, of course, quite inadequate 

 for biological purposes, but we need not reject it entirely, since for 

 all purposes of expression heredity is a general property of living 

 things, and with these there is even less reason than with crystals to 

 seek a cause in the function of a special organ. 



Inorganic elements and compounds are homogeneous and similar 

 in all masses or parts; but diversity is the rule among organisms, no 

 two of which are exact duplicates. The idea of a heredity which 

 maintains identity of structure or form represents no fact in nature. 

 The necessity of continued readjustment is general in life, and is not 

 confined, even in complex organisms, to preliminary stages or to re- 

 productive cells. The individual is not constant nor permanent, but 

 has its own cycle of growth, reproduction and decline, accompanied 

 by continuous changes in all parts of the bodily form and structure. 



* Chromosome differences utterly disproportional to the differences of the 

 adult organisms have recently been described by Monkhouse in hybrid fish eggs. 



