VARIATIONS DUE TO LAW 177 



raent, tend to produce variation. Slight variations 

 along one or more lines, continued and accelerated 

 from generation to generation, through long periods of 

 time, may produce very marked and permanent changes. 

 If the acquired characteristics, due to variation, have 

 been present for a long time, and the chief conditions 

 which produced the variation are continued, then such 

 acquired qualities are transmitted from parent to off- 

 spring, with a good degree of certainty; but perhaps 

 not so surely as are the characteristics which were 

 acquired earlier, when animals lived under less artificial 

 conditions. Happily, the variations either for better or 

 for worse, like those in the steel rule, are usually 

 within very narrow limits in a single generation or even 

 in several generations. If it were not so, one species 

 would merge into another, breeds and families be ex- 

 tinguished, and finally lost in one indistinguishable 

 whole. Variations in stock-breeding, like the errors 

 due to variations in the steel rule, if multiplied, often 

 may produce marked results. 



The St. Louis bridge, though accurately drafted and 

 accurately made, would not go together when moved 

 to another and warmer temperature than that in which 

 the pieces which constituted it were shaped. James B. 

 Eads did not lay this trouble to chance or to sport. 

 He knew that the lengthening of the girders of the 

 bridge was due to an inexorable law, and, taking 

 advantage of the law that governs expansion and 

 contraction of iron and steel, he shortened the girders 

 of the bridge by packing them in ice, thereby over- 

 coming the difficulty. 



