,8,, DR. W EI SM ANN'S THEORY OE HEREDITY. 179 



The evidence of the direct influence of tlie environment as 

 causing changes of structure is thus accumulative. Indeed, what is 

 surprising is not so much that changes are produced, as the constancy 

 of species in Nature ; for any pronounced alteration of habitat seems 

 almost always to set up alterations of structure. Such is the 

 experience of horticulturists, for when, for example, bulbous plants, 

 as tulips, &c., are first introduced from the East into English gardens, 

 they soon begin to change in all parts of their structure, so that in a 

 few years they are hardly recognisable.'^ 



3. Transformation due to direct Changes in the Germ-plasm. 



In maintaining the proposition that all hereditary structures have 

 arisen through changes first wrought in the germ-cells alone, and 

 must have been produced through the influences of external agencies 

 working directly upon the germ-plasm, Dr. Weismann seems to 

 have raised at least two very formidable and unnecessary difficulties 

 so far as plants are concerned. Firstly, the growth and development 

 of all vegetative structures takes place, and is mostly completed, 

 before any germ-cells are produced, the flowering process (which 

 alone contains them) invariably following the close of the period of 

 vegetative growth. Secondly, while the duration of the vegetative 

 period is relatively long, that of the existence of the germ-cells is 

 excessively short. Thus, no flowers last for more than a few hours 

 only. On the other hand, an annual requires at least several weeks 

 for its development ; perennial herbs and trees demand months and 

 years ; an aloe may be fifty years old before it blossoms, while ferns 

 and horsetails never have any germ-cells at all during the whole of 

 the agamic generation. 



If, however, we suppose with Strasburger that germ-plasm be 

 everywhere present and capable of being influenced by the environ- 

 ment, then these difficulties are at once removed, and the probability 

 is much increased that variations have come about through the 

 responsiveness of germ-plasm (assumed to accompany the protoplasm) 

 coupled with the power of fixing the eff"ects and of transmitting them 

 to the germ-cells on the appearance of the latter at the period of 

 flowering. 



Dr. Weismann's objection that " acquired characters cannot be 

 transmitted at all" (p. 267), because "they can never reach the 

 germ-cells " (p. 388), is thus met. For if germ-plasm be always 

 " unchanged," and capable of receiving and registering impressions 

 from without, wherever it may be, and not solely within the germ- 

 cells, as he seems to maintain, we thus have at least a sufficient 

 material. 



Here, again, we have admissions from him that this is con- 



!■' Such, e g., is the experience of Mr. Elwes. See also " Sur la production et la 

 fixation des Varietes dans les plantes d'ornement," by B. Verlot, 1865. 



N 2 



