February 1, 1895.] 



SCIENCE. 



V2l 



in SOUK' way an embodiment of all that is 

 to 1)0 unfolded out of it; while the other 

 school finds, in the stimulus which one part 

 of the segmenting egg or of the growing 

 organism exerts on other parts, the explan- 

 ation of each successive step in the process 

 of development. 



Advocates of these two views generally 

 regard themselves as opponents, but is there 

 any real antagonism ? 



We now have positive evidence enough 

 for each view to convince me that both are 

 true: that every change which takes place 

 in the organism from the l)eginning of seg- 

 mentation to the end of life is called forth 

 by some external stimulus either within the 

 body or without; and yet that the outcome 

 of the whole process of development is what 

 it is because it was all potential in the germ. 



The gun does not go off until the cap ex- 

 plodes: Imt it hits the mark because it is 

 aimed. 



While the distinction between the stimu- 

 lus to a vital change and the nature of the 

 change itself is obvious enougli in simple 

 cases, we may easily become confused and 

 lose sight of it in handling complicated 

 problems. 



A hen's egg does not develop without the 

 stimulus of heat, but the view that heat 

 cau.ses the chick is too grotesque for a sane 

 mind. 



\\'hat interests us is not that it becomes 

 a chick while a duck's egg in the same nest 

 becomes a duckling, but that the one gi'ows 

 into exquisite adjustment to the life of 

 fowls, while the otln'r becomes as admira- 

 bly adapted for the life of ducks. 



Here the stimulus comes from the exter- 

 nal world, but the case is just the same 

 when it is internal. 



The well-known results of castration 

 prove that the normal development of male 

 animals is dependent on some stimulus 

 which comes to the jmrts of the growing 

 body from the reproductive organs, but who 



can believe that this is an adequate expla- 

 nation of the short, sharp horns, the tliick 

 neck and the ferocity of the bull, or the 

 bright colors and higli courage of the cock ? 



The only explanation of the origin of these 

 useful structures worth considering is that 

 which attributes tliem to the retention by the 

 germ of the effects of past ages of selection. 



We have no reason to take a ditt'erent view 

 when the result varies with the stimulus. 

 Under one internal stimulus a bud becomes 

 a jelly-fish, while under others it may be- 

 come a liydranth, or a machopolyp or a blast- 

 ostyle, but the problem we have to solve in 

 tills case as in others is the origin of a beauti- 

 fully coordinated organism, with the distinct- 

 ive characters of its species, and with exquis- 

 ite fitness for a life like that of its ancestors. 



I showed some years ago that a small 

 crustacean, Alpheus heterochelis, develops 

 from the egg according to one plan at Beau- 

 fort in North Carolina, according to a sec- 

 ond at Key "\\^est in Florida, while it has 

 still a third life history at Nassau in the 

 Bahama Islands, but no one can beUeve 

 that the influences which cause this diver- 

 sity have anything to do with the final out- 

 come of the process. 



The case is exactly the same when a cell 

 which normally gives rise to a half or a 

 quarter of the body produces the whole un- 

 der a different stimulus. 



All the machinery in a gi-eat industrial 

 exposition may be started by a single elec- 

 tric contact, but however much the discov- 

 ery of the Ijutton may interest us, it helps 

 us little to understand the result. 



So it is with living orgiini.sm. Kxt^^rnal 

 conditions press the button, but it takes all 

 the inherited potency of living matter to do 

 the rest. 



It is an error to bi'lieve that great know- 

 ledge is needful for a clear grasp of fii-st 

 principles. Too often a great store of infor- 

 mation is like riches. " it cannot be .spared 

 nor left behind, but it hindereth the march ; 



