INDIVIDUAL DEVELOPMENT AND ANCESTRAL 

 DEVELOPMENT 



BY WILLIAM KEITH BROOKS 



[William Keith Brooks, Henry Walters Professor of Zoology, Johns Hopkins 

 University, b. Cleveland, Ohio, March 25, 1848. A.B. Williams College; LL.D. 

 ibid.; Ph.D. Harvard College; LL.D. Hpbart College. Associate Professor and 

 Professor, Johns Hopkins University, since 1877. Member of the National 

 Academy of Sciences; American Academy of Science; American Philosophical 

 Society. Author of Foundations of ZoSlogy; Scientific Results of the Voyage of 

 H.M.S. Cliallenger. Editor of Memoirs of Biological Laboratory, Johns Hopkins 

 University.] 



NEAR the end of the last century, zoologists of a speculative turn 

 were asking whether the changes that make up the history of species 

 are induced by the external world, or are inherent in germ-cells and 

 the living beings that arise from them. We had been told by Haeckel 

 that the inheritance of acquired characters is a necessary axiom of the 

 monistic creed, and, by Weismann, that acquired characters never 

 are, and never can be, inherited, because the architecture of germ- 

 plasm forbids. 



So the dispute went on, in the good old a priori way, with no sign 

 of any end except an armed truce, until wise and prudent zoologists 

 resolved to stop disputing and get to work. 



One of the fruits of this resolution was a wonderful series of obser- 

 vations and experiments upon the behavior of eggs and embryos 

 under abnormal conditions, the results of which are so instructive, 

 and so full of food for reflection, that they mark an epoch in the his- 

 tory of embryology, making one of its most notable chapters. In so 

 far, the resolution to cease from arguing, and to get to work, has been 

 altogether good, although one need read but little in the current 

 literature of embryology to find that we have not succeeded in laying 

 aside speculative questions. Many embryologists are now asking 

 whether the cell-differentiation which takes place during individual 

 development is inherent in eggs or their chromatin, or induced by the 

 interaction between the constituents of the egg, and between cell and 

 cell, and between the developing embryo and the external world. 



The dispute that was laid aside as vain and idle was whether the 

 development of species is inherent or acquired. The new question 

 is whether individual development is innate or superadded; but the 

 distinction is only a verbal one because that which is true of individ- 

 uals is also true of them when considered collectively. 



Some embryologists tell us that each cell that enters into the com- 

 position of a multicellular organism is a complete representative of 

 the species, having the same constitution and the same significance 



