"It would not be a true explanation, which is reserved for technical productions. But I can imagine that it would not be unprofitable to help one who is entirely ignorant on these questions by using makeshifts, in the form of allegories or analogies, which will serve as supports if he should take fright during the course of his earlier studies. These shocks are bound to occur, as, for instance, when he learns that a moving rigid rod undergoes contraction in the direction of motion."
"But this is proved to him!"
"Nevertheless, he does not easily accept it. For the general reader will say to himself: 'A superhuman effort is imposed on my mind. A rigid rod is the most constant of all things, and never before has one been compelled to regard something that is constant as variable.'"
"If he does not grasp it, no analogy will teach him."
"But perhaps it is possible. The analogy is to show him that the effort is not superhuman, and that thinking Man has already had occasion to become familiar with such transformations from constant to variable factors."
"I am afraid your analogy will prove a failure."
"From the scientific point of view this is probably true, inasmuch as all comparisons are imperfect, but the analogy may yet be of service as a last resort. For example, I should say to my general reader: 'Picture to yourself a savant of the Middle Ages who reflects on the constitution of animals and plants. One fact seems to him to be irrevocably true, namely, that the species are unchangeable! A palm tree is a palm tree, a horse is a horse, a worm a worm, and what is once a reptile remains a reptile. A species in itself denotes something absolutely invariant.'"
"The expression is wrong when taken in this connexion; you mean invariable."
"A little inaccuracy more or less does not affect the analogy. For the sake of my picture I should like to retain the conception-couple, variable and invariant. Well, then, the species give our savant the impression of invariance, as in the view that was held by Linné and Cuvier. This view necessarily has its counterpart in his thought. He argues that every species has its own original root, and that, in this sense, there is very extensive variation. The fundamental roots are extremely manifold; Nature has produced innumerable variations in her individual acts of creation. But now the Theory of Descent of Lamarck, Goethe, Oken, Geoffroy St. Hilaire, enters the field and produces a complete inversion of these two elements; the two parts of the earlier point of view change places. Our savant has to revise his whole world of thought. Now all organisms are to be traced back to a single original root: the latter, which was variable before, becomes an invariable unicellular primitive organism, but the apparently unchangeable species now becomes variable, in the widest possible sense. And even if this savant should exclaim: 'How am I to reconcile myself to this view?' his descendants later find no difficulty in accepting the idea that the organic roots are uniform, and that it is the species that are subject to all manner of variation as a compensating feature."
Einstein expressed himself very little pleased with this attempt at an analogy, and found that it was so far fetched that it could not be considered admissible.