When you use the argument from analogy, however, you must make sure that the similarity between the two cases runs to the point you wish to establish.  In the following extract from an argument in favor of commission government for all cities, the author explicitly limits his reasoning from the analogy of Washington to the point of the extension of the system to large cities.

If we look for successful governments by commission in this country, it is not difficult to find them in our largest cities.  The city of Washington is governed by a small commission, and is acknowledged to be one of our best-governed cities.  While this commission originated in an entirely different way from that of the commission form of government, successful administration under its rule is a valid answer to the argument that small commissions are suited only to the administration of small cities.[26]

Whenever you use this type of reasoning, it is wise thus to limit its bearing.  If in an argument in favor of allowing secret societies in a high school you rely on the analogy of college life, take pains to show that the resemblance covers the social life of a school.  If you were arguing that your city should establish a municipal gymnasium, and relied on the reasoning from the analogy of a family, in which all the members have a direct interest in the health of the others, show that this interest has practical grounds of welfare, and does not rest wholly on affection.  In every case, unless the limits of the analogy are obvious, specify them in order to carry your readers safely with you.

35.  False Analogy.  A peculiar danger of the argument from analogy is the fallacy which is known as false analogy, or reasoning to a conclusion which the similarity does not support.  Arguments in which there are many figures of speech, especially when the style is at all florid, are apt to slop over into this fallacy.  To liken education to the unfolding of a flower is all very well, if you do not go on to argue that because the lily of the field neither toils nor spins, therefore a child should do no work in school.  It is said that M. Stolypin, the late premier of Russia, once half apologized In the Duma for the slowness of his reforms, saying that he Was like a man shooting with a flintlock musket; to which one of the Liberal members replied that it was not a question of weapons, but of aim, and that if his Excellency was to go on shooting at the people, it would be better if he went on using flintlocks.  Under the auspices of the Carnegie Institution an expert in business administration made an inquiry into the methods of teaching and research in physics at various American universities, and made recommendations based on the conduct of business establishments.  A professor of physics in answer showed in how many ways the analogy between a business concern, whose end is profit, and a physical laboratory, whose end is the advancement of knowledge, is false and misleading.  The expert had suggested a general research board to correlate researches; the professor cited the cases of Airy, the astronomer royal of England, who by his dominating position held back astronomical research in England for a generation, and of Sir Humphry Davy, who discouraged the work of Faraday, when the latter was his assistant.