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The mind of man has shown great vigor and efficiency in developing ideas once they have come about. In the course of a single life-time the aeroplane has developed from a daring experiment by two bicycle mechanics to the most effective form of transport, whose convenience and efficiency are taken for granted. The radio has developed from a fragile miracle to a cheap commonplace. Development is something at which the mind excels: there are no conceivable limits to its excellence in this direction, which extends to the design of auxiliary electronic minds that can carry its developing capacity even farther.
What does 'which' in bold refer to? 'This direction' or 'its excellence (in this direction)'?
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Hi,
It's a bit unclear, but I think probably 'excellence'.

Clive
Comments  
Students: Are you brave enough to let our tutors analyse your pronunciation?
OK. Thanks, Clive!
I would agree with Clive.

"Development is something at which the mind excels: there are no conceivable limits to its (the minds) excellence in this direction, which ( the mind's excellence in development) extends to the design of auxiliary electronic minds that can carry it's developing capacity even farther."

Meaning that the mind excels so well in development that it is capable of creating an electronic mind that will further man's pursuit of developing new ideas.

-Ruru