The statement shows the theme that Balboa was a great explorer. This also introduces the theme that the new world was being dominated by Spain. These two themes are related because Balboa was a Spanish explorer, so his success in the new world meant the success of Spain.
You have not shown the book this question refers to. This prevents me from giving you a very specific answer. However, from the context of your question, it is possible to see that you are referring to a text that talks about Vasco Núñez de Balboa.
Following this context, it is possible to inform that:
In this case, the sentence “As he moves, the shadow of Spain moves with him” reinforces the theme of the success of Balboa's work as a Spanish explorer.
The regions that he explored, became the Spanish domain, allowing Spain to dominate a large part of Latin America and conquer all the riches present in that place.
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https://brainly.com/question/17333376?referrer=searchResults